Altair: Setting the size of mark_rect

[ilyasustun]

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]

You can do that with the rangeStep scale setting. Here's an example of setting it globally to 10 pixels:

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(-10, 10), 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(size=5).encode(
    x='x:O',
    y='y:O',
    color='z:Q'
).configure_scale(
    rangeStep=10
)

[ilyasustun]

Thank you so much Jake. When I try to add text on top of the cells it complains about Objects with "config" attribute cannot be used within LayerChart. Consider defining the config attribute in the LayerChart object instead.

heatmap = alt.Chart(df).mark_rect(cornerRadius=0).encode(
        alt.Y('rows:O', axis=alt.Axis(title='Model')),
        alt.X('bin:O',  axis=alt.Axis(title='Score')),
        alt.Color('score:Q', scale=alt.Scale(scheme='redblue'), legend=alt.Legend(title='%'))

    ).configure_scale(
        rangeStep=100
    )
#     ).properties(
#         width=1600,
#         height=300
#     )

#     # Configure text
    text = alt.Chart(df).mark_text(baseline='middle', color='black', size=15).encode(
        y='rows:O',
        x='bin:O',
        text='score:Q'
    )
    # Draw the chart
    chart = heatmap + text

    display(chart)

[ilyasustun]

Figured out from another answer of Jake:

    chart = alt.layer(heatmap, text).configure_view(
        stroke='transparent'
    ).configure_scale(
        rangeStep=100
    ).configure_axis(
        labelFontSize=16,
        titleFontSize=16,
        domainWidth=0.0
    )

[sbrugman]

@jakevdp Is there the option to set the size of mark_rect for each cell individually?
This post demonstrates how much more clarity that can bring for visualising correlations.

I was expecting this to work in a similar way to color:

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(-10, 10), 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',
    size='z:Q'
).configure_scale(
    rangeStep=10
)

[jakevdp]

You can do this with mark_square() - this is akin to mark_point() and mark_circle() in that it generates points that can have a size encoding, whereas rect generates rectangles whose size is controlled by x,x2,y,y2 encodings:

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(-10, 10), 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_square().encode(
    x='x:O',
    y='y:O',
    color='z:Q',
    size='z:Q'
)

[RoyalTS]

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(-10, 10), 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(size=5).encode(
    x='x:O',
    y='y:O',
    color='z:Q'
).configure_scale(
    rangeStep=10
)

This seems to have stopped working. I get

SchemaValidationError: Invalid specification

        altair.vegalite.v4.schema.core.ScaleConfig, validating 'additionalProperties'

        Additional properties are not allowed ('rangeStep' was unexpected)

[jakevdp]

rangeStep was removed in Altair v4.0; see https://altair-viz.github.io/releases/changes.html#grammar-changes