Basic Statistical Visualization

(This tutorial is adapted from Vega-Lite’s documentation)

This tutorial will guide you through the basic process of creating visualizations in Altair. First, you will need to make sure you have the Altair package and its dependencies installed (see Installation) and make sure you understand how altair plots are displayed (see Displaying Charts in Various Frontends). This tutorial will assume you are working within a Jupyter notebook user interface, so that plots are automatically rendered.

Here is the outline of this basic tutorial:

The Data

Data in Altair is built around the Pandas Dataframe. One of the defining characteristics of statistical visualization is that it begins with tidy Dataframes. For the purposes of this tutorial, we’ll start by importing Pandas and creating a simple DataFrame to visualize, with a categorical variable in column a and a numerical variable in column b:

import pandas as pd
data = pd.DataFrame({'a': list('CCCDDDEEE'),
                     'b': [2, 7, 4, 1, 2, 6, 8, 4, 7]})

When using Altair, datasets are most commonly provided as a Dataframe. As we will see, the labeled columns of the dataframe are an essential piece of plotting with Altair.

The Chart Object

The fundamental object in Altair is the Chart, which takes a dataframe as a single argument:

import altair as alt
chart = alt.Chart(data)

So far, we have defined the Chart object, but we have not yet told the chart to do anything with the data. That will come next.

Encodings and Marks

With this chart object in hand, we can now specify how we would like the data to be visualized. This is done via the mark attribute of the chart object, which is most conveniently accessed via the Chart.mark_* methods. For example, we can show the data as a point using mark_point():

alt.Chart(data).mark_point()

Here the rendering consists of one point per row in the dataset, all plotted on top of each other, since we have not yet specified positions for these points.

To visually separate the points, we can map various encoding channels, or channels for short, to columns in the dataset. For example, we could encode the variable a of the data with the x channel, which represents the x-axis position of the points. This can be done straightforwardly via the Chart.encode() method:

alt.Chart(data).mark_point().encode(
    x='a',
)

The encode() method builds a key-value mapping between encoding channels (such as x, y, color, shape, size, etc.) to columns in the dataset, accessed by column name.

For pandas dataframes, Altair automatically determines the appropriate data type for the mapped column, which in this case is a nominal value, or an unordered categorical.

Though we’ve now separated the data by one attribute, we still have multiple points overlapping within each category. Let’s further separate these by adding a y encoding channel, mapped to the "b" column:

alt.Chart(data).mark_point().encode(
    x='a',
    y='b'
)

The type of the data in the "b" column is again automatically-inferred by altair, and this time is treated as a quantitative type (i.e. real-valued). Additionally, we see that grid lines and appropriate axis titles are automatically added as well.

Data Transformation: Aggregation

To allow for more flexibility in how data are visualized, Altair has a built-in syntax for aggregation of data. For example, we can compute the average of all values by specifying this aggregate within the column identifier:

alt.Chart(data).mark_point().encode(
    x='a',
    y='average(b)'
)

Now within each x-axis category, we see a single point reflecting the average of the values within that category.

Typically, aggregated values are not represented by point markings, but by bar markings. We can do this by replacing mark_point() with mark_bar():

alt.Chart(data).mark_bar().encode(
    x='a',
    y='average(b)'
)

Because the categorical feature is mapped to the x-axis, the result is a vertical bar chart. To get a horizontal bar chart, all we need is to swap the x and y keywords:

alt.Chart(data).mark_bar().encode(
    y='a',
    x='average(b)'
)

Aside: Examining the JSON Output

Recall that Altair’s main purpose is to convert plot specifications to a JSON string that conforms to the Vega-Lite schema. It is instructive here to use the to_json() method to inspect the JSON specification that Altair is exporting and sending as JSON to Vega-Lite:

chart = alt.Chart(data).mark_bar().encode(
    x='a',
    y='average(b)',
)
print(chart.to_json())
{
  "$schema": "https://vega.github.io/schema/vega-lite/v2.4.3.json",
  "config": {
    "view": {
      "height": 300,
      "width": 400
    }
  },
  "data": {
    "values": [
      {
        "a": "C",
        "b": 2
      },
      {
        "a": "C",
        "b": 7
      },
      {
        "a": "C",
        "b": 4
      },
      {
        "a": "D",
        "b": 1
      },
      {
        "a": "D",
        "b": 2
      },
      {
        "a": "D",
        "b": 6
      },
      {
        "a": "E",
        "b": 8
      },
      {
        "a": "E",
        "b": 4
      },
      {
        "a": "E",
        "b": 7
      }
    ]
  },
  "encoding": {
    "x": {
      "field": "a",
      "type": "nominal"
    },
    "y": {
      "aggregate": "average",
      "field": "b",
      "type": "quantitative"
    }
  },
  "mark": "bar"
}

Notice here that encode(x='a') has been expanded to a JSON structure with a field name, and a type for the data. The encode(y='b') has been expanded similarly and includes an aggregate field.

Altair’s full shorthand syntax includes a way to specify the type of the column as well:

y = alt.Y('average(b):Q')
print(y.to_json())
{
  "aggregate": "average",
  "field": "b",
  "type": "quantitative"
}

This short-hand is equivalent to spelling-out the attributes by name:

y = alt.Y(field='b', type='quantitative', aggregate='average')
print(y.to_json())

This more verbose means of specifying channels can be used directly in Altair chart specifications, a fact that becomes useful when using some of the more advanced field configurations:

alt.Chart(data).mark_bar().encode(
    alt.Y('a', type='nominal'),
    alt.X('b', type='quantitative', aggregate='average')
)

Customizing your Visualization

By default, Altair via Vega-Lite makes some choices about default properties of the visualization. Altair also provides an API to customize the look of the visualization. For example, we can specify the axis titles using the axis attribute of channel classes, and we can specify the color of the marking by setting the color keyword of the Chart.mark_* methods to any valid HTML color string:

alt.Chart(data).mark_bar(color='firebrick').encode(
    alt.Y('a', axis=alt.Axis(title='category')),
    alt.X('average(b)', axis=alt.Axis(title='avg(b) by category'))
)

Publishing your Visualization

Once you have visualized your data, perhaps you would like to publish it somewhere on the web. This can be done straightforwardly using the Vega-Embed javascript package. A simple example of a stand-alone HTML document can be generated for any chart using the Chart.save() method:

chart = alt.Chart(data).mark_bar().encode(
    x='a',
    y='average(b)',
)
chart.save('chart.html')

The basic HTML template produces output that looks like this, where the JSON specification for your plot produced by Chart.to_json() should be stored in the vlSpec Javascript variable:

<!DOCTYPE html>
<html>
  <head>
    <script src="https://cdn.jsdelivr.net/npm/vega@3"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-lite@2"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-embed@3"></script>
  </head>
  <body>
    <div id="vis"></div>
    <script type="text/javascript">
      var spec = {};  /* JSON dump of your chart's spec */
      var opt = {"renderer": "canvas", "actions": false};  /* Options for the embedding */
      vegaEmbed("#vis", spec, opt);
    </script>
  </body>
</html>

The save() method provides a convenient way to save such HTML output to file. For more information on embedding Altair/Vega-Lite, see the documentation of the Vega-Embed project.