Add plot.barh in Series (#673)

This PR add series.plot.barh in Series.

Can be tested as below:

```python
import databricks.koalas as ks

kdf = ks.range(10)
kdf.to_pandas()['id'].plot.barh(colormap='Paired').figure.savefig("image1.png")
kdf['id'].plot.barh(colormap='Paired').figure.savefig("image2.png")
```

![image1](https://user-images.githubusercontent.com/6477701/63411569-c68b4100-c430-11e9-9d88-9f1113663def.png)

In case of this plot, we sample and match the row numbers around 1000.

```python
import databricks.koalas as ks

ks.range(1001)['id'].plot.barh(colormap='Paired').figure.savefig("image3.png")
```

![image3](https://user-images.githubusercontent.com/6477701/63411580-cc812200-c430-11e9-945b-0f74305c2507.png)


Partially addresses #665

databricks/koalas/plot.py

@@ -41,10 +41,11 @@ def _get_standard_kind(kind):
 
 if LooseVersion(pd.__version__) < LooseVersion('0.25'):
     from pandas.plotting._core import _all_kinds, BarPlot, BoxPlot, HistPlot, MPLPlot, PiePlot, \
-        AreaPlot, LinePlot
+        AreaPlot, LinePlot, BarhPlot
 else:
     from pandas.plotting._core import PlotAccessor
-    from pandas.plotting._matplotlib import BarPlot, BoxPlot, HistPlot, PiePlot, AreaPlot, LinePlot
+    from pandas.plotting._matplotlib import BarPlot, BoxPlot, HistPlot, PiePlot, AreaPlot, \
+        LinePlot, BarhPlot
     from pandas.plotting._matplotlib.core import MPLPlot
     _all_kinds = PlotAccessor._all_kinds
 
@@ -490,8 +491,38 @@ def _make_plot(self):
         super(KoalasLinePlot, self)._make_plot()
 
 
+class KoalasBarhPlot(BarhPlot):
+    max_rows = 1000
+
+    def __init__(self, data, **kwargs):
+        # 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.barh()
+        data = data.head(KoalasBarhPlot.max_rows + 1).to_pandas().to_frame()
+        self.partial = False
+        if len(data) > KoalasBarhPlot.max_rows:
+            self.partial = True
+            data = data.iloc[:KoalasBarhPlot.max_rows]
+        super(KoalasBarhPlot, self).__init__(data, **kwargs)
+
+    def _make_plot(self):
+        if self.partial:
+            self._get_ax(0).text(
+                1, 1, 'showing top 1,000 elements only', size=6, ha='right', va='bottom',
+                transform=self._get_ax(0).transAxes)
+            self.data = self.data.iloc[:KoalasBarhPlot.max_rows]
+
+        super(KoalasBarhPlot, self)._make_plot()
+
+
 _klasses = [
-    KoalasHistPlot, KoalasBarPlot, KoalasBoxPlot, KoalasPiePlot, KoalasAreaPlot, KoalasLinePlot]
+    KoalasHistPlot,
+    KoalasBarPlot,
+    KoalasBoxPlot,
+    KoalasPiePlot,
+    KoalasAreaPlot,
+    KoalasLinePlot,
+    KoalasBarhPlot,
+]
 _plot_klass = {getattr(klass, '_kind'): klass for klass in _klasses}
 
 
@@ -715,7 +746,38 @@ def bar(self, **kwds):
         return self(kind='bar', **kwds)
 
     def barh(self, **kwds):
-        return _unsupported_function(class_name='pd.Series', method_name='barh')()
+        """
+        Make a horizontal bar plot.
+
+        A horizontal bar plot is a plot that presents quantitative data with
+        rectangular bars with lengths proportional to the values that they
+        represent. A bar plot shows comparisons among discrete categories. One
+        axis of the plot shows the specific categories being compared, and the
+        other axis represents a measured value.
+
+        Parameters
+        ----------
+        x : label or position, default DataFrame.index
+            Column to be used for categories.
+        y : label or position, default All numeric columns in dataframe
+            Columns to be plotted from the DataFrame.
+        **kwds
+            Keyword arguments to pass on to :meth:`databricks.koalas.DataFrame.plot`.
+
+        Returns
+        -------
+        :class:`matplotlib.axes.Axes` or numpy.ndarray of them
+
+        See Also
+        --------
+        matplotlib.axes.Axes.bar : Plot a vertical bar plot using matplotlib.
+
+        Examples
+        --------
+        >>> df = ks.DataFrame({'lab':['A', 'B', 'C'], 'val':[10, 30, 20]})
+        >>> ax = df.val.plot.barh()
+        """
+        return self(kind='barh', **kwds)
 
     def box(self, **kwds):
         """

databricks/koalas/tests/test_series_plot.py

@@ -112,6 +112,27 @@ def test_line_plot(self):
         ax2 = kdf['a'].plot("line", colormap='Paired')
         self.compare_plots(ax1, ax2)
 
+    def test_barh_plot(self):
+        pdf = self.pdf1
+        kdf = self.kdf1
+
+        ax1 = pdf['a'].plot("barh", colormap='Paired')
+        ax2 = kdf['a'].plot("barh", colormap='Paired')
+        self.compare_plots(ax1, ax2)
+
+    def test_barh_plot_limited(self):
+        pdf = self.pdf2
+        kdf = self.kdf2
+
+        _, ax1 = plt.subplots(1, 1)
+        ax1 = pdf['id'][:1000].plot.barh(colormap='Paired')
+        ax1.text(1, 1, 'showing top 1,000 elements only', size=6, ha='right', va='bottom',
+                 transform=ax1.transAxes)
+        _, ax2 = plt.subplots(1, 1)
+        ax2 = kdf['id'].plot.barh(colormap='Paired')
+
+        self.compare_plots(ax1, ax2)
+
     def test_hist_plot(self):
         pdf = self.pdf1
         kdf = self.kdf1
@@ -210,7 +231,7 @@ def test_box_summary(self):
     def test_missing(self):
         ks = self.kdf1['a']
 
-        unsupported_functions = ['kde', 'barh']
+        unsupported_functions = ['kde']
         for name in unsupported_functions:
             with self.assertRaisesRegex(PandasNotImplementedError,
                                         "method.*Series.*{}.*not implemented".format(name)):

docs/source/reference/series.rst

@@ -331,6 +331,7 @@ specific plotting methods of the form ``Series.plot.<kind>``.
    Series.plot
    Series.plot.area
    Series.plot.bar
+   Series.plot.barh
    Series.plot.box
    Series.plot.hist
    Series.plot.line