Fix `by` parameter in pandas plotting

[datapythonista]

There is certain redundancy in pandas plotting, for example Series.hist() and Series.plot.hist().

In pandas-dev/pandas#28177 there is a proposal to make the API more consistent, and among other things to remove Series.hist() in favor of Series.plot.hist().

But as pointed out in the next tickets, they are not exactly the same:

pandas-dev/pandas#9274
pandas-dev/pandas#15079

Not sure if there are other differences, but before we can clean up the plotting API we should fix the by parameter in the plotting functions (hist and box). The by in already working in DataFrame.hist and DataFrame.boxplot for the comments in the issues. So, we just should copy what's in there to the needed DataFrame.plot. methods.

[martinagvilas]

Interesting! Would be happy to work on this, or split the hist and boxplot with somebody else if they want to attempt this as well (if that is possible).

[datapythonista]

Not sure, but probably to small to split it. But don't worry, I've got as many issues as people can want to work on. :)

Let me know if you need help.

[martinagvilas]

Great, I'll give it a go then.