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This website is for version 5. You can find the documentation for version 4 here.

Vega-Altair

Customizing Visualizations#

Altair’s goal is to automatically choose useful plot settings and configurations so that the user is free to think about the data rather than the mechanics of plotting. That said, once you have a useful visualization, you will often want to adjust certain aspects of it. This section of the documentation outlines some of the ways to make these adjustments.

Global Config vs. Local Config vs. Encoding#

There are often two or three different ways to specify the look of your plots depending on the situation. For example, suppose we are creating a scatter plot of the cars dataset:

import altair as alt
from vega_datasets import data
cars = data.cars.url

alt.Chart(cars).mark_point().encode(
    x='Acceleration:Q',
    y='Horsepower:Q'
)

Suppose you wish to change the color of the points to red, and the opacity of the points to 20%. There are three possible approaches to these:

  1. “Global Config” acts on an entire chart object

  2. “Local Config” acts on one mark of the chart

  3. “Encoding” channels can also be used to set some chart properties

Global Config#

First, every chart type has a "config" property at the top level that acts as a sort of theme for the whole chart and all of its sub-charts. Here you can specify things like axes properties, mark properties, selection properties, and more.

Altair allows you to access these through the configure_* methods of the chart. Here we will use the configure_mark() property:

alt.Chart(cars).mark_point().encode(
    x='Acceleration:Q',
    y='Horsepower:Q'
).configure_mark(
    opacity=0.2,
    color='red'
)

There are a couple things to be aware of when using this kind of global configuration:

  1. By design configurations will affect every mark used within the chart

  2. The global configuration is only permissible at the top-level; so, for example, if you tried to layer the above chart with another, it would result in an error.

For a full discussion of global configuration options, see Top-Level Chart Configuration.

Local Config#

If you would like to configure the look of the mark locally, such that the setting only affects the particular chart property you reference, this can be done via a local configuration setting.

In the case of mark properties, the best approach is to set the property as an argument to the mark_* method. Here we will use mark_point():

alt.Chart(cars).mark_point(opacity=0.2, color='red').encode(
    x='Acceleration:Q',
    y='Horsepower:Q'
)

Unlike when using the global configuration, here it is possible to use the resulting chart as a layer or facet in a compound chart.

Local config settings like this one will always override global settings.

Encoding#

Finally, it is possible to set chart properties via the encoding channel (see Encodings). Rather than mapping a property to a data column, you can map a property directly to a value using the value() function:

alt.Chart(cars).mark_point().encode(
    x='Acceleration:Q',
    y='Horsepower:Q',
    opacity=alt.value(0.2),
    color=alt.value('red')
)

Note that only a limited set of mark properties can be bound to encodings, so for some (e.g. fillOpacity, strokeOpacity, etc.) the encoding approach is not available.

Encoding settings will always override local or global configuration settings.

Which to Use?#

The precedence order for the three approaches is (from lowest to highest) global config, local config, encoding. That is, if a chart property is set both globally and locally, the local setting will win-out. If a property is set both via a configuration and an encoding, the encoding will win-out.

In most usage, we recommend always using the highest-precedence means of setting properties; i.e. an encoding, or a local configuration for properties that are not tied to an encoding. Global configurations should be reserved for creating themes that are applied just before the chart is rendered.

Adjusting the Title#

By default an Altair chart does not have a title, as seen in this example.

import altair as alt
from vega_datasets import data

iowa = data.iowa_electricity.url

alt.Chart(iowa).mark_area().encode(
    x="year:T",
    y=alt.Y("net_generation:Q").stack("normalize"),
    color="source:N"
)

You can add a simple title by passing the title keyword argument with the data.

alt.Chart(iowa, title="Iowa's green energy boom").mark_area().encode(
    x="year:T",
    y=alt.Y("net_generation:Q").stack("normalize"),
    color="source:N"
)

It is also possible to add a subtitle by passing in an alt.Title object.

alt.Chart(
   iowa,
   title=alt.Title(
       "Iowa's green energy boom",
       subtitle="A growing share of the state's energy has come from renewable sources"
   )
).mark_area().encode(
    x="year:T",
    y=alt.Y("net_generation:Q").stack("normalize"),
    color="source:N"
)

The subtitle can run to two lines by passing a list where each list item is a line (if you don’t want to create this list manually as in the example below, you can use the wrap function from the textwrap library to split a string into a list of substrings of a certain length).

alt.Chart(
   iowa,
   title=alt.Title(
       "Iowa's green energy boom",
       subtitle=["A growing share of the state's energy", "has come from renewable sources"]
   )
).mark_area().encode(
    x="year:T",
    y=alt.Y("net_generation:Q").stack("normalize"),
    color="source:N"
)

The Title object can also configure a number of other attributes, e.g., the position of the title and subtitle (see see Customizing Visualizations for details).

alt.Chart(
   iowa,
   title=alt.Title(
       "Iowa's green energy boom",
       subtitle=["A growing share of the state's energy", "has come from renewable sources"],
       anchor='start',
       orient='bottom',
       offset=20
   )
).mark_area().encode(
    x="year:T",
    y=alt.Y("net_generation:Q").stack("normalize"),
    color="source:N"
)

Adjusting Axis Limits#

The default axis limit used by Altair is dependent on the type of the data. To fine-tune the axis limits beyond these defaults, you can use the scale() method of the axis encodings. For example, consider the following plot:

import altair as alt
from vega_datasets import data

cars = data.cars.url

alt.Chart(cars).mark_point().encode(
    x='Acceleration:Q',
    y='Horsepower:Q'
)

Altair inherits from Vega-Lite the convention of always including the zero-point in quantitative axes; if you would like to turn this off, you can add the scale() method to the X encoding that specifies zero=False:

alt.Chart(cars).mark_point().encode(
    alt.X('Acceleration:Q').scale(zero=False),
    y='Horsepower:Q'
)

To specify exact axis limits, you can use the domain property of the scale:

alt.Chart(cars).mark_point().encode(
    alt.X('Acceleration:Q').scale(domain=(5, 20)),
    y='Horsepower:Q'
)

The problem is that the data still exists beyond the scale, and we need to tell Altair what to do with this data. One option is to “clip” the data by setting the "clip" property of the mark to True:

alt.Chart(cars).mark_point(clip=True).encode(
    alt.X('Acceleration:Q').scale(domain=(5, 20)),
    y='Horsepower:Q'
)

Another option is to “clamp” the data; that is, to move points beyond the limit to the edge of the domain:

alt.Chart(cars).mark_point().encode(
    alt.X('Acceleration:Q').scale(domain=(5, 20), clamp=True),
    y='Horsepower:Q'
).interactive()

For interactive charts like the one above, the clamping happens dynamically, which can be useful for keeping in mind outliers as you pan and zoom on the chart.

Adjusting Axis Labels#

Altair also gives you tools to easily configure the appearance of axis labels. For example consider this plot:

import pandas as pd
df = pd.DataFrame(
    {'x': [0.03, 0.04, 0.05, 0.12, 0.07, 0.15],
    'y': [10, 35, 39, 50, 24, 35]
})

alt.Chart(df).mark_circle().encode(
    x='x',
    y='y'
)

To fine-tune the formatting of the tick labels and to add a custom title to each axis, we can pass to the X and Y encoding a custom axis definition within the axis() method. Here is an example of formatting the x labels as a percentage, and the y labels as a dollar value:

alt.Chart(df).mark_circle().encode(
    alt.X('x').axis(format='%').title('percentage'),
    alt.Y('y').axis(format='$').title('dollar amount')
)

Axis labels can be easily removed:

alt.Chart(df).mark_circle().encode(
    alt.X('x').axis(labels=False),
    alt.Y('y').axis(labels=False)
)

Axis title can also be rotated:

alt.Chart(df).mark_circle().encode(
    alt.X('x').axis(title="x"),
    alt.Y('y').axis(
        title="Y Axis Title",
        titleAngle=0,
        titleAlign="left",
        titleY=-2,
        titleX=0,
    )
)

Additional formatting codes are available; for a listing of these see the d3 Format Code Documentation.

Adjusting the Legend#

A legend is added to the chart automatically when the color, shape or size arguments are passed to the encode() function. In this example we’ll use color.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color='species'
)

In this case, the legend can be customized by introducing the Color class and taking advantage of its legend() method. The shape and size arguments have their own corresponding classes.

The legend option on all of them expects a Legend object as its input, which accepts arguments to customize many aspects of its appearance. One example is to move the legend to another position with the orient argument.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color=alt.Color('species').legend(orient="left")
)

Another thing you can do is set a title; in this case we can use the title() method directly as a shortcut or specify the title parameter inside the legend() method:.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color=alt.Color('species').title("Species by color")
)

You can remove the legend entirely by submitting a null value.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color=alt.Color('species').legend(None),
)

Removing the Chart Border#

Basic Altair charts are drawn with both a grid and an outside border. To create a chart with no border, you will need to remove them both.

As an example, let’s start with a simple scatter plot.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color='species'
)

First remove the grid using the configure_axis() method.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color='species'
).configure_axis(
    grid=False
)

You’ll note that while the inside rules are gone, the outside border remains. Hide it by setting stroke=None inside configure_view() (strokeWidth=0 and strokeOpacity=0 also works):

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalWidth',
    y='petalLength',
    color='species'
).configure_axis(
    grid=False
).configure_view(
    stroke=None
)

It is also possible to completely remove all borders and axes by combining the above option with setting axis to None during encoding.

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    alt.X('petalWidth').axis(None),
    alt.Y('petalLength').axis(None),
    color='species'
).configure_axis(
    grid=False
).configure_view(
    stroke=None
)

Customizing Colors#

As discussed in Effect of Data Type on Color Scales, Altair chooses a suitable default color scheme based on the type of the data that the color encodes. These defaults can be customized using the scale() method of the Color class.

Color Schemes#

Altair includes a set of named color schemes for both categorical and sequential data, defined by the vega project; see the Vega documentation for a full gallery of available color schemes. These schemes can be passed to the scheme argument of the scale() method:

import altair as alt
from vega_datasets import data

cars = data.cars()

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color=alt.Color('Acceleration').scale(scheme="lightgreyred")
)

The color scheme we used above highlights points on one end of the scale, while keeping the rest muted. If we want to highlight the lower Acceleration data to red color instead, we can use the reverse parameter to reverse the color scheme:

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color=alt.Color('Acceleration').scale(scheme="lightgreyred", reverse=True)
)

Color Domain and Range#

To create a custom color scales, we can use the domain and range parameters of the scale method for the values and colors, respectively. This works both for continuous scales, where it can help highlight specific ranges of values:

domain = [5, 8, 10, 12, 25]
range_ = ['#9cc8e2', '#9cc8e2', 'red', '#5ba3cf', '#125ca4']

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color=alt.Color('Acceleration').scale(domain=domain, range=range_)
)

And for discrete scales:

domain = ['Europe', "Japan", "USA"]
range_ = ['seagreen', 'firebrick', 'rebeccapurple']

alt.Chart(cars).mark_point().encode(
    x='Horsepower',
    y='Miles_per_Gallon',
    color=alt.Color('Origin').scale(domain=domain, range=range_)
)

Raw Color Values#

The scale is what maps the raw input values into an appropriate color encoding for displaying the data. If your data entries consist of raw color names or codes, you can set scale(None) to use those colors directly:

import pandas as pd
import altair as alt

data = pd.DataFrame({
    'x': range(6),
    'color': ['red', 'steelblue', 'chartreuse', '#F4D03F', '#D35400', '#7D3C98']
})

alt.Chart(data).mark_point(
    filled=True,
    size=100
).encode(
    x='x',
    color=alt.Color('color').scale(None)
)

Adjusting the Width of Bar Marks#

The width of the bars in a bar plot are controlled through the size property in the mark_bar():

import altair as alt
import pandas as pd

data = pd.DataFrame({'name': ['a', 'b'], 'value': [4, 10]})

alt.Chart(data).mark_bar(size=10).encode(
    x='name:O',
    y='value:Q'
)

But since mark_bar(size=10) only controls the width of the bars, it might become possible that the width of the chart is not adjusted accordingly:

alt.Chart(data).mark_bar(size=30).encode(
    x='name:O',
    y='value:Q'
)

The width of the chart containing the bar plot can be controlled through setting the width property of the chart, either to a pixel value for any chart, or to a step value in the case of discrete scales.

Here is an example of setting the width to a single value for the whole chart:

alt.Chart(data).mark_bar(size=30).encode(
    x='name:O',
    y='value:Q'
).properties(width=200)

The width of the bars are set using mark_bar(size=30) and the width of the chart is set using properties(width=100)

Here is an example of setting the step width for a discrete scale:

alt.Chart(data).mark_bar(size=30).encode(
    x='name:N',
    y='value:Q'
).properties(width=alt.Step(100))

The width of the bars are set using mark_bar(size=30) and the width that is allocated for each bar bar in the chart is set using width=alt.Step(100)

Adjusting Chart Size#

The size of charts can be adjusted using the width and height properties. For example:

import altair as alt
from vega_datasets import data

cars = data.cars()

alt.Chart(cars).mark_bar().encode(
    x='Origin',
    y='count()'
).properties(
    width=200,
    height=150
)

Note that in the case of faceted or other compound charts, this width and height applies to the subchart rather than to the overall chart:

alt.Chart(cars).mark_bar().encode(
    x='Origin',
    y='count()',
    column='Cylinders:Q'
).properties(
    width=100,
    height=100
)

If you want your chart size to respond to the width of the HTML page or container in which it is rendered, you can set width or height to the string "container":

alt.Chart(cars).mark_bar().encode(
    x='Origin',
    y='count()',
).properties(
    width='container',
    height=200
)

Note that this will only scale with the container if its parent element has a size determined outside the chart itself; For example, the container may be a <div> element that has style width: 100%; height: 300px.

Chart Themes#

Altair makes available a theme registry that lets users apply chart configurations globally within any Python session. This is done via the alt.themes object.

The themes registry consists of functions which define a specification dictionary that will be added to every created chart. For example, the default theme configures the default size of a single chart:

>>> import altair as alt
>>> default = alt.themes.get()
>>> default()
{'config': {'view': {'continuousWidth': 300, 'continuousHeight': 300}}}

You can see that any chart you create will have this theme applied, and these configurations added to its specification:

import altair as alt
from vega_datasets import data

chart = alt.Chart(data.cars.url).mark_point().encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q'
)

chart.to_dict()

    {'config': {'view': {'continuousWidth': 300, 'continuousHeight': 300}}, 'data': {'url': 'https://cdn.jsdelivr.net/npm/vega-datasets@v1.29.0/data/cars.json'}, 'mark': {'type': 'point'}, 'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'}, 'y': {'field': 'Miles_per_Gallon', 'type': 'quantitative'}}, '$schema': 'https://vega.github.io/schema/vega-lite/v5.17.0.json'}

The rendered chart will then reflect these configurations:

chart

Changing the Theme#

If you would like to enable any other theme for the length of your Python session, you can call alt.themes.enable(theme_name). For example, Altair includes a theme in which the chart background is opaque rather than transparent:

alt.themes.enable('opaque')
chart.to_dict()

    {'config': {'background': 'white', 'view': {'continuousWidth': 300, 'continuousHeight': 300}}, 'data': {'url': 'https://cdn.jsdelivr.net/npm/vega-datasets@v1.29.0/data/cars.json'}, 'mark': {'type': 'point'}, 'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'}, 'y': {'field': 'Miles_per_Gallon', 'type': 'quantitative'}}, '$schema': 'https://vega.github.io/schema/vega-lite/v5.17.0.json'}

chart

Notice that the background color of the chart is now set to white. If you would like no theme applied to your chart, you can use the theme named 'none':

alt.themes.enable('none')
chart.to_dict()

    {'data': {'url': 'https://cdn.jsdelivr.net/npm/vega-datasets@v1.29.0/data/cars.json'}, 'mark': {'type': 'point'}, 'encoding': {'x': {'field': 'Horsepower', 'type': 'quantitative'}, 'y': {'field': 'Miles_per_Gallon', 'type': 'quantitative'}}, '$schema': 'https://vega.github.io/schema/vega-lite/v5.17.0.json'}

chart

Because the view configuration is not set, the chart is smaller than the default rendering.

If you would like to use any theme just for a single chart, you can use the with statement to enable a temporary theme:

with alt.themes.enable('default'):
    spec = chart.to_json()

Currently Altair does not offer many built-in themes, but we plan to add more options in the future.

Defining a Custom Theme#

The theme registry also allows defining and registering custom themes. A theme is simply a function that returns a dictionary of default values to be added to the chart specification at rendering time, which is then registered and activated.

For example, here we define a theme in which all marks are drawn with black fill unless otherwise specified:

import altair as alt
from vega_datasets import data

# define the theme by returning the dictionary of configurations
def black_marks():
    return {
        'config': {
            'view': {
                'height': 300,
                'width': 300,
            },
            'mark': {
                'color': 'black',
                'fill': 'black'
            }
        }
    }

# register the custom theme under a chosen name
alt.themes.register('black_marks', black_marks)

# enable the newly registered theme
alt.themes.enable('black_marks')

# draw the chart
cars = data.cars.url
alt.Chart(cars).mark_point().encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q'
)

If you want to restore the default theme, use:

alt.themes.enable('default')

For more ideas on themes, see the Vega Themes repository.

Localization#

The preferred format of numbers, dates, and currencies varies by language and locale. Vega-Altair takes advantage of D3’s localization support to make it easy to configure the locale for your chart using the global alt.renderers.set_embed_options function.

import altair as alt
alt.renderers.set_embed_options(
    format_locale=format_locale, time_format_locale=time_format_locale
)

Here format_locale and time_format_locale may either be D3 format dictionaries, or strings with the names of pre-defined locales. For example, here we use the Italian locale (named it-IT) for both currencies and dates:

import altair as alt
from vega_datasets import data

alt.renderers.set_embed_options(format_locale="it-IT", time_format_locale="it-IT")

source = data.stocks.url
chart = alt.Chart(source).mark_area().transform_filter('year(datum.date) == 2009').encode(
    x='date:T',
    y=alt.Y('price:Q', axis=alt.Axis(format="$.0f")),
    color='symbol:N'
)
chart
Area chart of stock prices using Italian locale

See https://unpkg.com/d3-format/locale/ for a list of available format locale names, and see https://unpkg.com/d3-time-format/locale/ for a list of available time format locales.

The configured localization settings persist upon saving.

Note

The globally defined properties, format_locale and time_format_locale, apply to the full session and are not specific to individual charts. To revert localization settings to the default U.S. English locale, use the following command:

alt.renderers.set_embed_options(format_locale="en-US", time_format_locale="en-US")
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