Having problem add text to grouped bar chart

Here’s what I’m trying to do .

I have a dataframe.

# init dataframe
df = pd.DataFrame({
    "group": ['g1','g1','g1','g2','g2','g2'],
    "type": ['a', 'b', 'c', 'a', 'b', 'c'],
    "value": [1,1,1,2,2,2]
})

# alt base
base = alt.Chart(df).encode(
    x='value',
    y='group',
    color='group:N',
    row='type'
)

Only plot bar.

base.make_bar

Only plot text.

base.mark_text().encode(text='value')

Add them up and raise error.

base.mark_text().encode(text='value') + base.mark_bar()

Trackback

ValueError                                Traceback (most recent call last)
<ipython-input-188-00e0014b2982> in <module>()
----> 1 base.mark_text().encode(text='value') + base.mark_bar()

~/Tools/anaconda3/lib/python3.6/site-packages/altair/vegalite/v3/api.py in __add__(self, other)
    481         if not isinstance(other, TopLevelMixin):
    482             raise ValueError("Only Chart objects can be layered.")
--> 483         return layer(self, other)
    484 
    485     def __and__(self, other):

~/Tools/anaconda3/lib/python3.6/site-packages/altair/vegalite/v3/api.py in layer(*charts, **kwargs)
   1877 def layer(*charts, **kwargs):
   1878     """layer multiple charts"""
-> 1879     return LayerChart(layer=charts, **kwargs)
   1880 
   1881 

~/Tools/anaconda3/lib/python3.6/site-packages/altair/vegalite/v3/api.py in __init__(self, data, layer, **kwargs)
   1826         for spec in layer:
   1827             _check_if_valid_subspec(spec, 'LayerChart')
-> 1828             _check_if_can_be_layered(spec)
   1829         super(LayerChart, self).__init__(data=data, layer=list(layer), **kwargs)
   1830         self.data, self.layer = _combine_subchart_data(self.data, self.layer)

~/Tools/anaconda3/lib/python3.6/site-packages/altair/vegalite/v3/api.py in _check_if_can_be_layered(spec)
   1664         for channel in ['row', 'column', 'facet']:
   1665             if _get(encoding, channel) is not Undefined:
-> 1666                 raise ValueError("Faceted charts cannot be layered.")
   1667     if isinstance(spec, (Chart, LayerChart)):
   1668         return

ValueError: Faceted charts cannot be layered.

What I did wrong here?

[jakevdp]

Sure, there’s a section about it in the docs here: https://altair-viz.github.io/user_guide/compound_charts.html

Let me know if anything there could be clarified more.

[jakevdp]

You’re getting this error because faceted charts cannot be layered (This is because in general there is no guarantee that the facets will align). However, layered charts can be faceted, and so you can accomplish what you want by calling the facet() method on a layered chart. For example:

import altair as alt
import pandas as pd

# init dataframe
df = pd.DataFrame({
    "group": ['g1','g1','g1','g2','g2','g2'],
    "type": ['a', 'b', 'c', 'a', 'b', 'c'],
    "value": [1,1,1,2,2,2]
})

# alt base
base = alt.Chart(df).encode(
    x='value',
    y='group',
    color='group:N',
)

alt.layer(
  base.mark_bar(),
  base.mark_text(dx=5).encode(text='value')
).facet(
  row='type'
)