Unified PlotAccessor for DataFrame and Series

[DumbMachine]

• Ready for review

Aims to fix #1661
New PlotAccessor works the same way as the old.

TODO:

• Confirm each function has appropriate example and options in the docs

Examples:

import pandas as pd
import databricks.koalas as ks
import matplotlib.pyplot as plt
kf =  ks.DataFrame([[1, 2, 3, 4], [5, 6, 7, 8]], columns=["A", "B", "C", "D"])

# dataframe like
print(kf.plot)
kf.plot(kind="line")
kf.plot(kind="line", backend="plotly", width=400, height=400).show()

<databricks.koalas.plot.PlotAccessor object at 0x7f32d669b590>

# series like
print(kf.A.plot)
kf.A.plot(kind="line");
kf.A.plot(kind="line", backend="plotly", width=400, height=400).show()

<databricks.koalas.plot.PlotAccessor object at 0x7f32cdffd750>

Error are handled, same as before

kf.plot(kind="pie", legend=False)
kf.plot(kind="pie",  backend="plotly", width=400, height=400).show()

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-13-9597ff26ce78> in <module>
      1 kf.plot(kind="pie", legend=False)
----> 2 kf.plot(kind="pie",  backend="plotly", width=400, height=400).show()

~/work/koalas/databricks/koalas/plot.py in __call__(self, kind, backend, **kwargs)
   1322 
   1323         if plot_backend.__name__ != "databricks.koalas.plot":
-> 1324             return plot_backend.plot(plot_data, kind=kind, **kwds)
   1325 
   1326         if kind not in self._all_kinds:

~/anaconda3/lib/python3.7/site-packages/plotly/__init__.py in plot(data_frame, kind, **kwargs)
    101         return histogram(data_frame, **new_kwargs)
    102     raise NotImplementedError(
--> 103         "kind='%s' not yet supported for plotting.backend='plotly'" % kind
    104     )
    105 

NotImplementedError: kind='pie' not yet supported for plotting.backend='plotly'

And finally

# exists for dataframe like
kf.plot(kind="scatter", x='A', y='B')
# does not exist for series like
kf.A.plot(kind="scatter")

---------------------------------------------------------------------------
PandasNotImplementedError                 Traceback (most recent call last)
<ipython-input-6-674f15f3badc> in <module>
----> 1 kf.A.new_plot(kind="scatter")

~/work/koalas/databricks/koalas/new_plot.py in __call__(self, kind, backend, **kwargs)
    157         if isinstance(self.data, Series):
    158             if kind not in self._series_kinds:
--> 159                 return  unsupported_function(class_name="pd.Series", method_name=kind)()
    160             return plot_series(data=self.data, kind=kind, **kwds)
    161         elif isinstance(self.data, DataFrame):

~/work/koalas/databricks/koalas/missing/__init__.py in unsupported_function(*args, **kwargs)
     21     def unsupported_function(*args, **kwargs):
     22         raise PandasNotImplementedError(
---> 23             class_name=class_name, method_name=method_name, reason=reason
     24         )
     25 

PandasNotImplementedError: The method `pd.Series.scatter()` is not implemented yet.

[DumbMachine]

Could anybody help me with the PIP tests error? I do not get any error during local testing.

[ueshin]

I submitted a PR to you branch DumbMachine#2.

[ueshin]

@DumbMachine Could you push an empty commit to run tests again?
Seems like the current build doesn't include the new changes for some reason.

[ueshin]

Otherwise, LGTM.
@DumbMachine Great work!

[ueshin]

@HyukjinKwon What do you think the doc should be if we consolidate the plotting classes?

[HyukjinKwon]

Haven't built it and checked by myself but I think it's okay to consolidate in general.

[ueshin]

Could you move the example for DataFrame?

[ueshin]

@DumbMachine Could you address my comments above?

[DumbMachine]

Sorry for the delay, have made the changes. I will modify if any further changes are required,

[ueshin]

Thanks! merging.