Grouped barchart for <= 12 rows

.gitignore

@@ -102,3 +102,8 @@ venv.bak/
 
 # mypy
 .mypy_cache/
+
+
+ # specific
+examples/_matplotlib.ipynb
+.vscode/*

altair_pandas/_core.py

@@ -1,6 +1,8 @@
 import altair as alt
 import pandas as pd
 
+CHART_GROUPED_MAX = 12  # TODO: arbitrary - for discussion
+
 
 def _valid_column(column_name):
     return str(column_name)
@@ -97,6 +99,9 @@ def _preprocess_data(self, with_index=True, usecols=None):
             if isinstance(data.index, pd.MultiIndex):
                 data.index = pd.Index(
                     [str(i) for i in data.index], name=data.index.name)
+
+            if data.index.name is None:
+                data.index.name = 'index'
             return data.reset_index()
         return data
 
@@ -136,13 +141,30 @@ def area(self, x=None, y=None, **kwargs):
 
     # TODO: bars should be grouped, not stacked.
     def bar(self, x=None, y=None, **kwargs):
-        return self._xy(
+
+        chart = self._xy(
             {'type': 'bar', 'orient': 'vertical'}, x, y, **kwargs)
 
+        if len(self._data) <= CHART_GROUPED_MAX:
+
+            return chart.encode(
+                x=alt.X('column:N', title=None),
+                column=x or self._data.index.name or 'index'
+            )
+
+        return chart
+
     def barh(self, x=None, y=None, **kwargs):
         chart = self._xy(
             {'type': 'bar', 'orient': 'horizontal'}, x, y, **kwargs)
         chart.encoding.x, chart.encoding.y = chart.encoding.y, chart.encoding.x
+
+        if len(self._data) <= CHART_GROUPED_MAX:
+            return chart.encode(
+                y=alt.Y('column:N', title=None),
+                row=x or self._data.index.name or 'index'
+            )
+
         return chart
 
     def scatter(self, x, y, c=None, s=None, **kwargs):

altair_pandas/test_plotting.py

@@ -13,6 +13,11 @@ def dataframe():
     return pd.DataFrame({'x': range(5), 'y': range(5)})
 
 
+@pytest.fixture
+def large_dataframe():
+    return pd.DataFrame({'x': range(20), 'y': range(20)})
+
+
 @pytest.mark.parametrize('data', [
     pd.Series(
         range(6),
@@ -27,9 +32,13 @@ def test_multiindex(data, with_plotting_backend):
     chart = data.plot.bar()
     spec = chart.to_dict()
     assert list(chart.data.iloc[:, 0]) == [str(i) for i in data.index]
-    assert spec['encoding']['x']['field'] == 'index'
     assert spec['encoding']['x']['type'] == 'nominal'
 
+    if isinstance(data, pd.Series):
+        assert spec['encoding']['x']['field'] == 'index'
+    else:
+        assert spec['encoding']['x']['field'] == 'column'
+
 
 def test_nonstring_column_names(with_plotting_backend):
     data = pd.DataFrame(np.ones((3, 4)))
@@ -65,9 +74,11 @@ def test_dataframe_basic_plot(dataframe, kind, with_plotting_backend):
     spec = chart.to_dict()
     if kind == 'bar':
         assert spec['mark'] == {'type': 'bar', 'orient': 'vertical'}
+        assert spec['encoding']['x']['field'] == 'column'
     else:
         assert spec['mark'] == kind
-    assert spec['encoding']['x']['field'] == 'index'
+        assert spec['encoding']['x']['field'] == 'index'
+
     assert spec['encoding']['y']['field'] == 'value'
     assert spec['encoding']['color']['field'] == 'column'
     assert spec['transform'][0]['fold'] == ['x', 'y']
@@ -85,6 +96,17 @@ def test_dataframe_barh(dataframe, with_plotting_backend):
     chart = dataframe.plot.barh()
     spec = chart.to_dict()
     assert spec['mark'] == {'type': 'bar', 'orient': 'horizontal'}
+    assert spec['encoding']['y']['field'] == 'column'
+    assert spec['encoding']['x']['field'] == 'value'
+    assert spec['encoding']['color']['field'] == 'column'
+    assert spec['transform'][0]['fold'] == ['x', 'y']
+    assert spec['encoding']['row']['field'] == 'index'
+
+
+def test_dataframe_large_barh(large_dataframe, with_plotting_backend):
+    chart = large_dataframe.plot.barh()
+    spec = chart.to_dict()
+    assert spec['mark'] == {'type': 'bar', 'orient': 'horizontal'}
     assert spec['encoding']['y']['field'] == 'index'
     assert spec['encoding']['x']['field'] == 'value'
     assert spec['encoding']['color']['field'] == 'column'