I'm working off the code in this example, slightly modified:
https://altair-viz.github.io/gallery/layered_heatmap_text.html
I'm trying to figure out why the my code breaks when I try to facet into columns.
# Import data
import altair as alt
from vega_datasets import data
source = data.cars()
# Configure common options
base = alt.Chart(source)
scale = alt.Scale(paddingInner=0)
The original version works fine:
# Configure heatmap
heatmap = base.mark_rect().encode(
alt.X('Cylinders:O', scale=scale),
alt.Y('Year:O', scale=scale),
color='count()'
)
# Configure text
text = base.mark_text(baseline='middle').encode(
x='Cylinders:O',
y='Year:O',
text='count()',
color=alt.value('white')
)
# Draw the chart
heatmap+text
Now I would like to facet by Origin
. The code below works when I display heatmap
and text
separately, but when I combine them, I get an error.
# Configure heatmap
heatmap = base.mark_rect().encode(
alt.X('Cylinders:O', scale=scale),
alt.Y('Year:O', scale=scale),
color='count()',
column = 'Origin'
)
# Configure text
text = base.mark_text(baseline='middle').encode(
x='Cylinders:O',
y='Year:O',
text='count()',
color=alt.value('white'),
column = 'Origin'
)
# Draw the chart
heatmap+text
Here's the error message - I'm not really understanding where the issue is stemming from.
---------------------------------------------------------------------------
SchemaValidationError Traceback (most recent call last)
~/miniconda3/lib/python3.6/site-packages/altair/vegalite/v2/api.py in _repr_mimebundle_(self, include, exclude)
1111 try:
-> 1112 dct = self.to_dict()
1113 except Exception:
~/miniconda3/lib/python3.6/site-packages/altair/vegalite/v2/api.py in to_dict(self, *args, **kwargs)
420 kwargs['validate'] = 'deep'
--> 421 dct = super(TopLevelMixin, copy).to_dict(*args, **kwargs)
422
~/miniconda3/lib/python3.6/site-packages/altair/utils/schemapi.py in to_dict(self, validate, ignore, context)
253 except jsonschema.ValidationError as err:
--> 254 raise SchemaValidationError(self, err)
255 return result
<class 'str'>: (<class 'TypeError'>, TypeError('sequence item 1: expected str instance, int found',))
During handling of the above exception, another exception occurred:
TypeError Traceback (most recent call last)
~/miniconda3/lib/python3.6/site-packages/IPython/core/formatters.py in __call__(self, obj, include, exclude)
968
969 if method is not None:
--> 970 return method(include=include, exclude=exclude)
971 return None
972 else:
~/miniconda3/lib/python3.6/site-packages/altair/vegalite/v2/api.py in _repr_mimebundle_(self, include, exclude)
1112 dct = self.to_dict()
1113 except Exception:
-> 1114 utils.display_traceback(in_ipython=True)
1115 return {}
1116 else:
~/miniconda3/lib/python3.6/site-packages/altair/utils/core.py in display_traceback(in_ipython)
403
404 if ip is not None:
--> 405 ip.showtraceback(exc_info)
406 else:
407 traceback.print_exception(*exc_info)
~/miniconda3/lib/python3.6/site-packages/IPython/core/interactiveshell.py in showtraceback(self, exc_tuple, filename, tb_offset, exception_only, running_compiled_code)
2036 value, tb, tb_offset=tb_offset)
2037
-> 2038 self._showtraceback(etype, value, stb)
2039 if self.call_pdb:
2040 # drop into debugger
~/miniconda3/lib/python3.6/site-packages/ipykernel/zmqshell.py in _showtraceback(self, etype, evalue, stb)
544 u'traceback' : stb,
545 u'ename' : unicode_type(etype.__name__),
--> 546 u'evalue' : py3compat.safe_unicode(evalue),
547 }
548
~/miniconda3/lib/python3.6/site-packages/ipython_genutils/py3compat.py in safe_unicode(e)
63 """
64 try:
---> 65 return unicode_type(e)
66 except UnicodeError:
67 pass
~/miniconda3/lib/python3.6/site-packages/altair/utils/schemapi.py in __unicode__(self)
67 schema_path = ['{}.{}'.format(cls.__module__, cls.__name__)]
68 schema_path.extend(self.schema_path)
---> 69 schema_path = '->'.join(val for val in schema_path[:-1]
70 if val not in ('properties',
71 'additionalProperties',
TypeError: sequence item 1: expected str instance, int found
The issue is that faceted charts cannot be layered (this is because in general, there is no guarantee that the two layers will have compatible facets). On the other hand, layered charts can be faceted. For example:
# Import data
import altair as alt
from vega_datasets import data
source = data.cars()
# Configure common options
scale = alt.Scale(paddingInner=0)
# Configure heatmap
heatmap = alt.Chart().mark_rect().encode(
alt.X('Cylinders:O', scale=scale),
alt.Y('Year:O', scale=scale),
color='count()'
)
# Configure text
text = alt.Chart().mark_text(baseline='middle').encode(
x='Cylinders:O',
y='Year:O',
text='count()',
color=alt.value('white')
)
# Draw the chart.
alt.layer(heatmap, text, data=source).facet(
column='Origin'
)
Note that when using this approach, it's important to specify the data at the level of the facet rather than in the child layers. For more information, see Faceted Charts in the Altair docs.
The error message here used to be more helpful... it seems that the jsonschema
library is reporting schema validation errors differently than it used to.
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