Stacked charts in composite charts

It would be great to have a feature like this within composite charts.

With this change, you can choose between grouped charts, as it is now in composed charts, and stacked charts, as in the "single" charts. The charts can be mixed in any way. I think the "old" charts are still working the same.

To enable this functionality, I added a "_stacked" version of each chart, which can be selected under "Series type."

In bar charts you can now build different towers and put in a line with additional informations.

Or with area charts I use the lines to show the totals, min and max.

P.S. very cool extension. I was able to replace my old excel chart, where I every year forget how they are working.

(I don't know, how to remove the first commit, sorry)