python - Altair Ridgeline doesn't create a plot with nominal groups

Question

I try to create a Ridgeline plot in altair. Let's assume my dataframe consists of str and float columns:

a     object
b    float64
dtype: object

with values like

    a       b
0  25  2303.0
1  29  2676.0
2  18  2983.0
3  16  1489.0
4  21  3468.0

I used code from Altair gallery to create my chart: https://altair-viz.github.io/gallery/ridgeline_plot.html. My code with changed data and column names:

import pandas as np
import numpy as np

source = pd.DataFrame(columns=list('ab'))
source['a'] = np.random.randint(0,17,size=500)
source['a'] = source['a'].astype('str')
source['b'] = np.random.randint(1000,5000,size=500).astype('float')

import altair as alt

step = 20
overlap = 1

alt.Chart(source, height=step).transform_joinaggregate(
    mean_temp='mean(b)', groupby=['a']
).transform_bin(
    ['bin_max', 'bin_min'], 'b'
).transform_aggregate(
    value='count()', groupby=['a', 'b', 'bin_min', 'bin_max']
).transform_impute(
    impute='value', groupby=['a', 'b'], key='bin_min', value=0
).mark_area(
    interpolate='monotone',
    fillOpacity=0.8,
    stroke='lightgray',
    strokeWidth=0.5
).encode(
    alt.X('bin_min:Q', bin='binned', title=''),
    alt.Y(
        'value:Q',
        scale=alt.Scale(range=[step, -step * overlap]),
        axis=None
    ),
    alt.Fill(
        'b:Q',
        legend=None,
    )
).facet(
    row=alt.Row(
        'a:T',
        title=None,
        header=alt.Header(labelAngle=0, labelAlign='right', format='%B')
    )
).properties(
    title='',
    bounds='flush'
).configure_facet(
    spacing=0
).configure_view(
    stroke=None
).configure_title(
    anchor='end'
)

When I use row=alt.Row('a:T'...) it thinks my data is temporal month, but works fine:

But when I change type there to nominal 'a:N', result is empty. How to fix it?

question from:https://stackoverflow.com/questions/65837215/altair-ridgeline-doesnt-create-a-plot-with-nominal-groups

Answer 1

For chart rendering errors like this, often there are clues in the browser's developer console. In this case the following error is reported:

vega@5?noext:1 ERROR Error: invalid format: %B

I believe that "%B" is not a valid format code for nominal data. If you remove format='%B' from the header, the nominal row encoding will work.