Support ability to create native gr.Barplots with multiple series side-by-side

[abidlabs]

I wanted to create a gr.Barplot that plots multiple y columns for each x value, but it seems like this is not possible with our gr.Barplot. We do support the ability to stack bars like this:

But not have them side by side. The API I would expect is to be able to pass a list of columns for the y parameter, not just a single column name