Multiple grouped charts with altair - dataframe

Question

My data has 4 attributes: dataset (D1/D2), model (M1/M2), layer (L1/L2), scene (S1/S2). I can make a chart grouped by scenes and then merge plots horizontally and vertically (pic above).
However, I would like to have 'double grouping' by scene and dataset, like merging the D1 and D2 plots by placing blue/orange bars from next to each other but with different opacity or pattern/hatch.
Basically something like this (pretend that the black traits are a hatch pattern).
Here is the code to reproduce the first plot
import numpy as np
import itertools
import argparse
import pandas as pd
import matplotlib.pyplot as plt
import os
import altair as alt
alt.renderers.enable('altair_viewer')
np.random.seed(0)
################################################################################
model_keys = ['M1', 'M2']
data_keys = ['D1', 'D2']
scene_keys = ['S1', 'S2']
layer_keys = ['L1', 'L2']
ys = []
models = []
dataset = []
layers = []
scenes = []
for sc in scene_keys:
for m in model_keys:
for d in data_keys:
for l in layer_keys:
for s in range(10):
data_y = list(np.random.rand(10) / 10)
ys += data_y
scenes += [sc] * len(data_y)
models += [m] * len(data_y)
dataset += [d] * len(data_y)
layers += [l] * len(data_y)
# ------------------------------------------------------------------------------
df = pd.DataFrame({'Y': ys,
'Model': models,
'Dataset': dataset,
'Layer': layers,
'Scenes': scenes})
bars = alt.Chart(df, width=100, height=90).mark_bar().encode(
# field to group columns on
x=alt.X('Scenes:N',
title=None,
axis=alt.Axis(
grid=False,
title=None,
labels=False,
),
),
# field to use as Y values and how to calculate
y=alt.Y('Y:Q',
aggregate='mean',
axis=alt.Axis(
grid=True,
title='Y',
titleFontWeight='normal',
),
),
# field to use for sorting
order=alt.Order('Scenes',
sort='ascending',
),
# field to use for color segmentation
color=alt.Color('Scenes',
legend=alt.Legend(orient='bottom',
padding=-10,
),
title=None,
),
)
error_bars = alt.Chart(df).mark_errorbar(extent='ci').encode(
x=alt.X('Scenes:N'),
y=alt.Y('Y:Q'),
)
text = alt.Chart(df).mark_text(align='center',
baseline='line-bottom',
color='black',
dy=-5 # y-shift
).encode(
x=alt.X('Scenes:N'),
y=alt.Y('mean(Y):Q'),
text=alt.Text('mean(Y):Q', format='.1f'),
)
chart_base = bars + error_bars + text
chart_base = chart_base.facet(
# field to use to use as the set of columns to be represented in each group
column=alt.Column('Layer:N',
# header=alt.Header(
# labelFontStyle='bold',
# ),
title=None,
sort=list(set(models)), # get unique indices
),
spacing={"row": 0, "column": 15},
)
def unique(sequence):
seen = set()
return [x for x in sequence if not (x in seen or seen.add(x))]
for i, m in enumerate(unique(models)):
chart_imnet = chart_base.transform_filter(
alt.FieldEqualPredicate(field='Dataset', equal='D1'),
).transform_filter(
alt.FieldEqualPredicate(field='Model', equal=m)
)
chart_places = chart_base.transform_filter(
alt.FieldEqualPredicate(field='Dataset', equal='D2')
).transform_filter(
alt.FieldEqualPredicate(field='Model', equal=m)
)
if i == 0:
title_params = dict({'align': 'center', 'anchor': 'middle', 'dy': -10})
chart_imnet = chart_imnet.properties(title=alt.TitleParams('D1', **title_params))
chart_places = chart_places.properties(title=alt.TitleParams('D2', **title_params))
chart_places = alt.concat(chart_places,
title=alt.TitleParams(
m,
baseline='middle',
orient='right',
anchor='middle',
angle=90,
# dy=10,
dx=30 if i == 0 else 0,
),
)
if i == 0:
chart = (chart_imnet | chart_places).resolve_scale(x='shared')
else:
chart = (chart & (chart_imnet | chart_places).resolve_scale(x='shared'))
chart.save('test.html')

Answer 1

For now, I don't know a good answer, but once https://github.com/altair-viz/altair/pull/2528 is accepted you can use the xOffset encoding channel as such:
alt.Chart(df, height=90).mark_bar(tooltip=True).encode(
x=alt.X("Scenes:N"),
y=alt.Y("mean(Y):Q"),
color=alt.Color("Scenes:N"),
opacity=alt.Opacity("Dataset:N"),
xOffset=alt.XOffset("Dataset:N"),
column=alt.Column('Layer:N'),
row=alt.Row("Model:N")
).resolve_scale(x='independent')
Which will result in:
See Colab Notebook or Vega Editor
EDIT
To control the opacity and legend names one can do as such
alt.Chart(df, height=90).mark_bar(tooltip=True).encode(
x=alt.X("Scenes:N"),
y=alt.Y("mean(Y):Q"),
color=alt.Color("Scenes:N"),
opacity=alt.Opacity("Dataset:N",
scale=alt.Scale(domain=['D1', 'D2'],
range=[0.2, 1.0]),
legend=alt.Legend(labelExpr="datum.label == 'D1' ? 'D1 - transparent' : 'D2 - full'")),
xOffset=alt.XOffset("Dataset:N"),
column=alt.Column('Layer:N'),
row=alt.Row("Model:N")
).resolve_scale(x='independent')