ENH: Add bar plot for DataFrame (#695)

kdf:

<img width="348" alt="Screen Shot 2019-08-26 at 2 36 11 PM" src="https://user-images.githubusercontent.com/9269816/63691312-21e17880-c80f-11e9-83d3-cb87655a383f.png">

pdf:

<img width="327" alt="Screen Shot 2019-08-26 at 2 36 21 PM" src="https://user-images.githubusercontent.com/9269816/63691323-2ad24a00-c80f-11e9-91ba-c2c1ec7589a4.png">

databricks/koalas/plot.py

@@ -54,9 +54,16 @@ class TopNPlot:
     max_rows = 1000
 
     def get_top_n(self, data):
+        from databricks.koalas import DataFrame, Series
         # Simply use the first 1k elements and make it into a pandas dataframe
         # For categorical variables, it is likely called from df.x.value_counts().plot.xxx().
-        data = data.head(TopNPlot.max_rows + 1).to_pandas().to_frame()
+        if isinstance(data, Series):
+            data = data.head(TopNPlot.max_rows + 1).to_pandas().to_frame()
+        elif isinstance(data, DataFrame):
+            data = data.head(TopNPlot.max_rows + 1).to_pandas()
+        else:
+            ValueError("Only DataFrame and Series are supported for plotting.")
+
         self.partial = False
         if len(data) > TopNPlot.max_rows:
             self.partial = True
@@ -633,7 +640,7 @@ def plot_series(data, kind='line', ax=None,                    # Series unique
 
 def _plot(data, x=None, y=None, subplots=False,
           ax=None, kind='line', **kwds):
-
+    from databricks.koalas import DataFrame
     # function copied from pandas.plotting._core
     # and adapted to handle Koalas DataFrame and Series
 
@@ -642,6 +649,15 @@ def _plot(data, x=None, y=None, subplots=False,
         klass = _plot_klass[kind]
     else:
         raise ValueError("%r is not a valid plot kind" % kind)
+
+    # check data type and do preprocess before applying plot
+    if isinstance(data, DataFrame):
+        if x is not None:
+            data = data.set_index(x)
+        # TODO: check if value of y is plottable
+        if y is not None:
+            data = data[y]
+
     plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
     plot_obj.generate()
     plot_obj.draw()
@@ -949,8 +965,27 @@ def pie(self, bw_method=None, ind=None, **kwds):
     def area(self, bw_method=None, ind=None, **kwds):
         return _unsupported_function(class_name='pd.DataFrame', method_name='area')()
 
-    def bar(self, bw_method=None, ind=None, **kwds):
-        return _unsupported_function(class_name='pd.DataFrame', method_name='bar')()
+    def bar(self, x=None, y=None, **kwds):
+        """
+        Vertical bar plot.
+
+        Parameters
+        ----------
+        x : label or position, optional
+            Allows plotting of one column versus another.
+            If not specified, the index of the DataFrame is used.
+        y : label or position, optional
+            Allows plotting of one column versus another.
+            If not specified, all numerical columns are used.
+        `**kwds` : optional
+            Additional keyword arguments are documented in
+            :meth:`Koalas.DataFrame.plot`.
+
+        Returns
+        -------
+        axes : :class:`matplotlib.axes.Axes` or numpy.ndarray of them
+        """
+        return self(kind='bar', x=x, y=y, **kwds)
 
     def barh(self, bw_method=None, ind=None, **kwds):
         return _unsupported_function(class_name='pd.DataFrame', method_name='barh')()

databricks/koalas/tests/test_frame_plot.py

@@ -49,10 +49,35 @@ def test_line_plot(self):
         ax4 = kdf.plot.line(colormap='Paired')
         self.compare_plots(ax3, ax4)
 
+    def test_bar_plot(self):
+        pdf = self.pdf1
+        kdf = self.kdf1
+
+        ax1 = pdf.plot(kind='bar', colormap='Paired')
+        ax2 = kdf.plot(kind='bar', colormap='Paired')
+        self.compare_plots(ax1, ax2)
+
+        ax3 = pdf.plot.bar(colormap='Paired')
+        ax4 = kdf.plot.bar(colormap='Paired')
+        self.compare_plots(ax3, ax4)
+
+    def test_bar_with_x_y(self):
+        # this is testing plot with specified x and y
+        pdf = pd.DataFrame({'lab': ['A', 'B', 'C'], 'val': [10, 30, 20]})
+        kdf = koalas.from_pandas(pdf)
+
+        ax1 = pdf.plot(kind="bar", x='lab', y='val', colormap='Paired')
+        ax2 = kdf.plot(kind="bar", x='lab', y='val', colormap='Paired')
+        self.compare_plots(ax1, ax2)
+
+        ax3 = pdf.plot.bar(x='lab', y='val', colormap='Paired')
+        ax4 = kdf.plot.bar(x='lab', y='val', colormap='Paired')
+        self.compare_plots(ax3, ax4)
+
     def test_missing(self):
         ks = self.kdf1
 
-        unsupported_functions = ['area', 'bar', 'barh', 'box', 'density', 'hexbin',
+        unsupported_functions = ['area', 'barh', 'box', 'density', 'hexbin',
                                  'hist', 'kde', 'pie', 'scatter']
         for name in unsupported_functions:
             with self.assertRaisesRegex(PandasNotImplementedError,

docs/source/reference/frame.rst

@@ -233,3 +233,4 @@ specific plotting methods of the form ``DataFrame.plot.<kind>``.
 
    DataFrame.plot
    DataFrame.plot.line
+   DataFrame.plot.bar