Altair: Adjust marker width in heat map (mark_rect) ?

[apahl]

Hi,

is it possible to adjust the width of the rectangles in a mark_rect() heat map, so that they are not square anymore?

This would be useful for cases where there are many more values on the x axis than on the y axis (as in a phenotypic profile (my use case)).
Many thanks in advance for your help.

Kind regards,
Axel

[jakevdp]

The easiest way to do this is to explicitly set the width and height of the chart; e.g.

import altair as alt
import numpy as np
import pandas as pd

# Compute x^2 + y^2 across a 2D grid
x, y = np.meshgrid(range(-20, 20), range(-5, 5))
z = x ** 2 + y ** 2

# Convert this grid to columnar data expected by Altair
source = pd.DataFrame({'x': x.ravel(),
                     'y': y.ravel(),
                     'z': z.ravel()})

alt.Chart(source).mark_rect().encode(
    x='x:O',
    y='y:O',
    color='z:Q'
).properties(
    width=400,
    height=200
)

[apahl]

Cool, that totally works!

Thanks a lot for the quick and illustrative answer!

Awesome project.

Kind regards,
Axel

[ilyasustun]

Well, my question is regarding setting the size (width and height) of each of the cells in mark_rect(). How can we do that such that the size is fixed, and the overall heatmap just gets bigger if we have more boxes to show?

[jakevdp]

(For future searches, this last question is answered here: https://github.com/altair-viz/altair/issues/1618#issuecomment-513424086)