ENH: Implement frame plot

[charlesdong1991]

This PR is mainly creating frame plot which is kind of base method, now, only line plot is added.

koalas plot:

pandas plot:

[codecov-io]

Codecov Report

Merging #686 into master will increase coverage by 0.15%.
The diff coverage is 97.5%.

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
+ Coverage   93.52%   93.68%   +0.15%     
==========================================
  Files          32       32              
  Lines        5455     5492      +37     
==========================================
+ Hits         5102     5145      +43     
+ Misses        353      347       -6

Impacted Files	Coverage Δ
databricks/koalas/frame.py	94.74% <100%> (+0.01%)	⬆️
databricks/koalas/plot.py	94.86% <97.29%> (+0.58%)	⬆️
databricks/koalas/internal.py	96.15% <0%> (ø)	⬆️
databricks/koalas/indexes.py	97.19% <0%> (+5.6%)	⬆️

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[HyukjinKwon]

Can we mimic pandas documenation (https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.plot.line.html#pandas.DataFrame.plot.line)? It's okay to don't include the images for now.

[HyukjinKwon]

Thanks @charlesdong1991.

• Can you add a test to test missing plot implementations? See test_series_plot.py

      def test_missing(self):
      ks = self.kdf1['a']
  
      unsupported_functions = ['kde']
      for name in unsupported_functions:
          with self.assertRaisesRegex(PandasNotImplementedError,
                                      "method.*Series.*{}.*not implemented".format(name)):
              getattr(ks.plot, name)()

• Can you show an example code with output? (see Add plot.area in Series #670)

• We should create and update plot in documentation (see https://github.com/databricks/koalas/pull/670/files#diff-9cc87824986b3722f2133fdb0023fb69R332). Let's match it to https://pandas.pydata.org/pandas-docs/stable/reference/frame.html#plotting

[charlesdong1991]

Hi, @HyukjinKwon thanks for taking a look at the PR, and I changed the code to reflect your points in the review. And here I attach the output with the test example code.

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[HyukjinKwon]

Looks fine except, do you mind if I ask to post pandas example (input/output) too in PR description as well? Just want to make sure that it shows the same chart.

[charlesdong1991]

thanks for your feedback, fair enough! I added it in the description. Once it's merged, then I will add other sub modules for frame plot. @HyukjinKwon

[HyukjinKwon]

Merged to master.

Thanks you for addressing my comments.