Altair: Space between rectangles / bars?

[breadbaron]

Sorry if this is covered elsewhere, but I wasn't able to find it anywhere. When using mark_bar() or mark_rect() to create a heatmap, is it possible to remove the spacing between rectangles/bars?

In my case I'm plotting a 2-D density and the rectangle boundaries distract from the density.
thanks!

[jakevdp]

It looks like the size property is not applied to rect or bar marks. But one way you can do this is to encode both the fill and the stroke, and make the stroke fill the space between marks. For example:

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

x, y = np.meshgrid(np.arange(30), np.arange(30))
z = np.exp(-(x - 14)**2 / (5 ** 2) - (y - 22) ** 2 / (5 ** 2))
data = pd.DataFrame({
    'x': x.ravel(),
    'y': y.ravel(),
    'z': z.ravel()
})

alt.Chart(data).mark_rect(strokeWidth=3).encode(
  x='x:O',
  y='y:O',
  fill='z:Q',
  stroke='z:Q'
)

[breadbaron]

Thanks for looking into that @jakevdp . This solution works great

[mattijn]

A better solution would be to adjust the inner padding for x and y band-ordinal scales using the bandPaddingInner, which defaults to 0.1 (docs).

alt.Chart(data).mark_rect().encode(
  x='x:O',
  y='y:O',
  fill='z:Q'
).configure_scale(bandPaddingInner=0)

[jakevdp]

Thanks @mattjin – I didn't know about that configuration.

[caleb-lindgren]

Just a quick note for anyone else who comes across this--the bandPaddingInner solution didn't work for me, but the fill and stroke solution worked fine. (I guess there was some change in the code, or there's something different about my plot.)