Remove varname from legend in plotppc

[MarcoGorelli]

closes #1540

Description

Remove varname from legend in plotppc. Motivation is:

• it's already the label of the x-axis
• if the case of multiple plots, then a single legend is still valid

xref #1540 (comment)

Of course the current behavior is problematic, because we have one legend that is supposed to work for both plots but is referring only to the first. So I suggest to have as default one legend and remove from the legend the name of the variable.

Here's what the output from examples/matplotlib/mpl_plot_ppc.py would now look like - note how obs only appears as the label of the x-axis, rather than being repeated in the legend

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[OriolAbril]

Matplotlib looks good, thanks for the test! My only comment is boke, does bokeh not show the var names in the legend already?

[MarcoGorelli]

Thanks @OriolAbril

I'm not familiar with bokeh, but running the same code with backend='bokeh' gives:

so it looks like the Bokeh ppc plots are fine as they are?

[OriolAbril]

I think the legend should be present, especially thinking about begginers who use the bokeh backend, I don't think we can assume everyone will know what do each of the colors represent. I have seen my fair share of people confused between the observed kde and the pp mean kde for example. We can open an issue to fix that in some other PR if you prefer too and merge this as is.

[MarcoGorelli]

Sure, have opened #1563 for that (I'd prefer to keep it separate as it feels different to the issue which this PR addresses)