Interactivity and Selections¶
Altair’s interactivity and grammar of selections are one of its unique features among available plotting libraries. In this section, we will walk through the variety of selection types that are available, and begin to practice creating interactive charts and dashboards.
There are three basic types of selections available:
Interval Selection:
alt.selection_interval()Single Selection:
alt.selection_single()Multi Selection:
alt.selection_multi()
And we will cover four basic things that you can do with these selections
Conditional encodings
Scales
Filters
Domains
import altair as alt
from vega_datasets import data
Basic Interactions: Panning, Zooming, Tooltips¶
The basic interactions that Altair makes available are panning, zooming, and tooltips.
This can be done in your chart without any use of the selection interface, using the
interactive() shortcut method and the tooltip encoding.
For example, with our standard cars dataset, we can do the following:
cars = data.cars()
cars.head()
| Name | Miles_per_Gallon | Cylinders | Displacement | Horsepower | Weight_in_lbs | Acceleration | Year | Origin | |
|---|---|---|---|---|---|---|---|---|---|
| 0 | chevrolet chevelle malibu | 18.0 | 8 | 307.0 | 130.0 | 3504 | 12.0 | 1970-01-01 | USA |
| 1 | buick skylark 320 | 15.0 | 8 | 350.0 | 165.0 | 3693 | 11.5 | 1970-01-01 | USA |
| 2 | plymouth satellite | 18.0 | 8 | 318.0 | 150.0 | 3436 | 11.0 | 1970-01-01 | USA |
| 3 | amc rebel sst | 16.0 | 8 | 304.0 | 150.0 | 3433 | 12.0 | 1970-01-01 | USA |
| 4 | ford torino | 17.0 | 8 | 302.0 | 140.0 | 3449 | 10.5 | 1970-01-01 | USA |
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color='Origin',
tooltip='Name'
).interactive()
At this point, hovering over a point will bring up a tooltip with the name of the car model, and clicking/dragging/scrolling will pan and zoom on the plot.
More Sophisticated Interaction: Selections¶
Basic Selection Example: Interval¶
As an example of a selection, let’s add an interval selection to a chart.
We’ll start with our cannonical scatter plot:
cars = data.cars()
cars.head()
| Name | Miles_per_Gallon | Cylinders | Displacement | Horsepower | Weight_in_lbs | Acceleration | Year | Origin | |
|---|---|---|---|---|---|---|---|---|---|
| 0 | chevrolet chevelle malibu | 18.0 | 8 | 307.0 | 130.0 | 3504 | 12.0 | 1970-01-01 | USA |
| 1 | buick skylark 320 | 15.0 | 8 | 350.0 | 165.0 | 3693 | 11.5 | 1970-01-01 | USA |
| 2 | plymouth satellite | 18.0 | 8 | 318.0 | 150.0 | 3436 | 11.0 | 1970-01-01 | USA |
| 3 | amc rebel sst | 16.0 | 8 | 304.0 | 150.0 | 3433 | 12.0 | 1970-01-01 | USA |
| 4 | ford torino | 17.0 | 8 | 302.0 | 140.0 | 3449 | 10.5 | 1970-01-01 | USA |
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color='Origin'
)
To add selection behavior to a chart, we create the selection object and use the add_selection method:
interval = alt.selection_interval()
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color='Origin'
).add_selection(
interval
)
This adds an interaction to the plot that lets us select points on the plot; perhaps the most common use of a selection is to highlight points by conditioning their color on the result of the selection.
This can be done with alt.condition:
interval = alt.selection_interval()
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(interval, 'Origin', alt.value('lightgray'))
).add_selection(
interval
)
The alt.condition function takes three arguments: a selection object, a value to be applied to points within the selection, and a value to be applied to points outside the selection.
Here we use alt.value('lightgray') to make certain that the color is treated as an actual color, rather than the name of a data column.
Customizing the Interval selection¶
The alt.selection_interval() function takes a number of additional arguments; for example, by specifying encodings, we can control whether the selection covers x, y, or both:
interval = alt.selection_interval(encodings=['x'])
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(interval, 'Origin', alt.value('lightgray'))
).add_selection(
interval
)
interval = alt.selection_interval(encodings=['y'])
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(interval, 'Origin', alt.value('lightgray'))
).add_selection(
interval
)
The empty argument lets us control whether empty selections contain all values, or none of the values;
with empty='none' points are grayed-out by default:
interval = alt.selection_interval(empty='none')
alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(interval, 'Origin', alt.value('lightgray'))
).add_selection(
interval
)
Single Selections¶
The alt.selection_single() function allows the user to click on single chart objects to select them, one at a time.
We’ll make the points a bit bigger so they are easier to click:
single = alt.selection_single()
alt.Chart(cars).mark_circle(size=100).encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(single, 'Origin', alt.value('lightgray'))
).add_selection(
single
)
The single selection allows other behavior as well; for example, we can set nearest=True and on='mouseover' to update the highlight to the nearest point as we move the mouse:
single = alt.selection_single(on='mouseover', nearest=True)
alt.Chart(cars).mark_circle(size=100).encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(single, 'Origin', alt.value('lightgray'))
).add_selection(
single
)
Multi Selection¶
The alt.selection_multi() function is quite similar to the single function, except it lets multiple points be selected at once, while holding the shift key:
multi = alt.selection_multi()
alt.Chart(cars).mark_circle(size=100).encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(multi, 'Origin', alt.value('lightgray'))
).add_selection(
multi
)
Options like on and nearest also work for multi selections:
multi = alt.selection_multi(on='mouseover', nearest=True)
alt.Chart(cars).mark_circle(size=100).encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color=alt.condition(multi, 'Origin', alt.value('lightgray'))
).add_selection(
multi
)
Selection Binding¶
Above we have seen how alt.condition can be used to bind the selection to different aspects of the chart.
Let’s look at a few other ways that a selection can be used:
Binding Scales¶
For an interval selection, another thing you can do with the selection is bind the selection region to the chart scales:
bind = alt.selection_interval(bind='scales')
alt.Chart(cars).mark_circle(size=100).encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
color='Origin:N'
).add_selection(
bind
)