Add layout support in plot_frame similar to the hist_frame layout

[pascal-schetelat]

Add an optional layout keyword argument in order to set the subplots layout.
If none provided, falls back to default behavior.

[jreback]

• This needs a couple of tests which test passing an invalid layout.
• Pls add a mention in release notes (refereing this PR #)

[jorisvandenbossche]

You will also have to expose this in plot_frame (https://github.com/pascal-schetelat/pandas/blob/master/pandas/tools/plotting.py#L1827) to make it usable in df.plot()

[pascal-schetelat]

@jreback I duplicated the same tests on the layout as in hist_frame in _get_layout. Do you think additional tests are required?

@jorisvandenbossche you mean adding an entry in the doc or explicitely passing a layout argument in plot_frame?

[jorisvandenbossche]

@pascal-schetelat Yes, at least adding it to the docs.
I also meant to explicitly pass it as a argument, but I see that there are also others that are passed through the *kwds, so I am not really sure about this what is best.

[jreback]

@pascal-schetelat this will close #6306, yes?

[jreback]

@pascal-schetelat pls rebase and push again, your changes are not showing up here

[sinhrks]

I think the shape of returned axes should be cared. hist returns ndarray which has the same shape as the specified layout, not flattened. Personally whichever possible if there is a policy.