Implement (DataFrame|Series).plot.pie in plotly (#1971)

This PR implements `DataFrame.plot.pie` in plotly as below:

```python
from databricks import koalas as ks
kdf = ks.DataFrame(
    {'a': [1, 2, 3, 4, 5, 6],
     'b': [100, 200, 300, 400, 500, 600]},
    index=[10, 20, 30, 40, 50, 60])
ks.options.plotting.backend = 'plotly'
kdf.plot.pie(y="b")
```

![Screen Shot 2020-12-17 at 3 28 12 PM](https://user-images.githubusercontent.com/6477701/102451779-87005380-407c-11eb-85f3-aa2d8e62c991.png)

Binder to test: https://mybinder.org/v2/gh/HyukjinKwon/koalas/plotly-pie?filepath=docs%2Fsource%2Fgetting_started%2F10min.ipynb

Author: HyukjinKwon

databricks/koalas/plot/core.py

@@ -173,6 +173,12 @@ def _get_plot_backend(backend=None):
             return KoalasPlotAccessor._backends[backend]
 
         module = KoalasPlotAccessor._find_backend(backend)
+
+        if backend == "plotly":
+            from databricks.koalas.plot.plotly import plot_plotly
+
+            module.plot = plot_plotly(module.plot)
+
         KoalasPlotAccessor._backends[backend] = module
         return module
 
@@ -714,7 +720,7 @@ def area(self, x=None, y=None, **kwds):
         elif isinstance(self.data, DataFrame):
             return self(kind="area", x=x, y=y, **kwds)
 
-    def pie(self, y=None, **kwds):
+    def pie(self, **kwds):
         """
         Generate a pie plot.
 
@@ -728,7 +734,7 @@ def pie(self, y=None, **kwds):
         ----------
         y : int or label, optional
             Label or position of the column to plot.
-            If not provided, ``subplots=True`` argument must be passed.
+            If not provided, ``subplots=True`` argument must be passed (matplotlib-only).
         **kwds
             Keyword arguments to pass on to :meth:`Koalas.Series.plot`.
 
@@ -764,9 +770,15 @@ def pie(self, y=None, **kwds):
             return self(kind="pie", **kwds)
         else:
             # pandas will raise an error if y is None and subplots if not True
-            if isinstance(self.data, DataFrame) and y is None and not kwds.get("subplots", False):
-                raise ValueError("pie requires either y column or 'subplots=True'")
-            return self(kind="pie", y=y, **kwds)
+            if (
+                isinstance(self.data, DataFrame)
+                and kwds.get("y", None) is None
+                and not kwds.get("subplots", False)
+            ):
+                raise ValueError(
+                    "pie requires either y column or 'subplots=True' (matplotlib-only)"
+                )
+            return self(kind="pie", **kwds)
 
     def scatter(self, x, y, **kwds):
         """

databricks/koalas/plot/plotly.py

@@ -0,0 +1,51 @@
+#
+# Copyright (C) 2019 Databricks, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+import pandas as pd
+
+
+def plot_plotly(origin_plot):
+    def plot(data, kind, **kwargs):
+        # Koalas specific plots
+        if kind == "pie":
+            return plot_pie(data, **kwargs)
+
+        # Other plots.
+        return origin_plot(data, kind, **kwargs)
+
+    return plot
+
+
+def plot_pie(data, **kwargs):
+    from plotly import express
+
+    if isinstance(data, pd.Series):
+        pdf = data.to_frame()
+        return express.pie(pdf, values=pdf.columns[0], names=pdf.index, **kwargs)
+    elif isinstance(data, pd.DataFrame):
+        # DataFrame
+        values = kwargs.pop("y", None)
+        default_names = None
+        if values is not None:
+            default_names = data.index
+
+        return express.pie(
+            data,
+            values=kwargs.pop("values", values),
+            names=kwargs.pop("names", default_names),
+            **kwargs
+        )
+    else:
+        raise RuntimeError("Unexpected type: [%s]" % type(data))

databricks/koalas/tests/plot/test_frame_plot_plotly.py

@@ -18,6 +18,7 @@
 
 import pandas as pd
 import numpy as np
+from plotly import express
 
 from databricks import koalas as ks
 from databricks.koalas.config import set_option, reset_option
@@ -143,3 +144,33 @@ def check_scatter_plot(pdf, kdf, x, y, c):
         pdf1 = pd.DataFrame(np.random.rand(50, 4), columns=["a", "b", "c", "d"])
         kdf1 = ks.from_pandas(pdf1)
         check_scatter_plot(pdf1, kdf1, x="a", y="b", c="c")
+
+    def test_pie_plot(self):
+        def check_pie_plot(kdf):
+            pdf = kdf.to_pandas()
+            self.assertEqual(
+                kdf.plot(kind="pie", y=kdf.columns[0]),
+                express.pie(pdf, values="a", names=pdf.index),
+            )
+
+            self.assertEqual(
+                kdf.plot(kind="pie", values="a"), express.pie(pdf, values="a"),
+            )
+
+        kdf1 = self.kdf1
+        check_pie_plot(kdf1)
+
+        # TODO: support multi-index columns
+        # columns = pd.MultiIndex.from_tuples([("x", "y"), ("y", "z")])
+        # kdf1.columns = columns
+        # check_pie_plot(kdf1)
+
+        # TODO: support multi-index
+        # kdf1 = ks.DataFrame(
+        #     {
+        #         "a": [1, 2, 3, 4, 5, 6, 7, 8, 9, 15, 50],
+        #         "b": [2, 3, 4, 5, 7, 9, 10, 15, 34, 45, 49]
+        #     },
+        #     index=pd.MultiIndex.from_tuples([("x", "y")] * 11),
+        # )
+        # check_pie_plot(kdf1)

databricks/koalas/tests/plot/test_series_plot_plotly.py

@@ -17,6 +17,7 @@
 from distutils.version import LooseVersion
 
 import pandas as pd
+from plotly import express
 
 from databricks import koalas as ks
 from databricks.koalas.config import set_option, reset_option
@@ -98,3 +99,32 @@ def test_area_plot(self):
 
         # just a sanity check for df.col type
         self.assertEqual(pdf.sales.plot(kind="area"), kdf.sales.plot(kind="area"))
+
+    def test_pie_plot(self):
+        kdf = self.kdf1
+        pdf = kdf.to_pandas()
+        self.assertEqual(
+            kdf["a"].plot(kind="pie"), express.pie(pdf, values=pdf.columns[0], names=pdf.index),
+        )
+
+        # TODO: support multi-index columns
+        # columns = pd.MultiIndex.from_tuples([("x", "y")])
+        # kdf.columns = columns
+        # pdf.columns = columns
+        # self.assertEqual(
+        #     kdf[("x", "y")].plot(kind="pie"),
+        #     express.pie(pdf, values=pdf.iloc[:, 0].to_numpy(), names=pdf.index.to_numpy()),
+        # )
+
+        # TODO: support multi-index
+        # kdf = ks.DataFrame(
+        #     {
+        #         "a": [1, 2, 3, 4, 5, 6, 7, 8, 9, 15, 50],
+        #         "b": [2, 3, 4, 5, 7, 9, 10, 15, 34, 45, 49]
+        #     },
+        #     index=pd.MultiIndex.from_tuples([("x", "y")] * 11),
+        # )
+        # pdf = kdf.to_pandas()
+        # self.assertEqual(
+        #     kdf["a"].plot(kind="pie"), express.pie(pdf, values=pdf.columns[0], names=pdf.index),
+        # )