How to do annotations with Altair?

Question

I am trying to write some text inside the figure to highlight something in my plot (equivalent to 'annotate' in matplotlib). Any idea? Thanks

Answer 1

You can get annotations into your Altair plots in two steps:

1. Use mark_text() to specify the annotation's position, fontsize etc.
2. Use transform_filter() from datum to select the points (data subset) that needs the annotation. Note the line from altair import datum.

Code:

import altair as alt
from vega_datasets import data
alt.renderers.enable('notebook')

from altair import datum #Needed for subsetting (transforming data)


iris = data.iris()

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

annotation = alt.Chart(iris).mark_text(
    align='left',
    baseline='middle',
    fontSize = 20,
    dx = 7
).encode(
    x='petalLength',
    y='petalWidth',
    text='petalLength'
).transform_filter(
    (datum.petalLength >= 5.1) & (datum.petalWidth < 1.6)
)


points + annotation

which produces:

These are static annotations. You can also get interactive annotations by binding selections to the plots.