ENH: Group some columns in subplots with DataFrame.plot
`df.plot(subplots=True)`  will create one subplot per column. Is there a way to group multiple columns on the same subplot (and leave the rest of the column separated)?

I'd be happy to submit a PR if that's something you'd consider? In terms of API  `subplot` could accept a list of tuples where each tuple indicates which columns should be grouped together.