Allowing widgets to select columns to add to views

[rickmcgeer]

A common feature in dashboarding packages is to allow widgets to select datasets to be included in a plot. We don't permit this; the datasets to be included are the columns of a View, and the columns are statically chosen when the View is built. All Views do is filter data.
The reason we did this was because the type of chart we can support is dependent on the columns, but this is true only for a few specialized chart types. Most charts (Line, Area, Bar) support 1-n datasets, and these can be easily added and taken away dynamically. So the proposal is to have columns dynamically added to views (at the price of having some views specialized to specific chart types, but that is true anyway). So...

1. The x axis dataset must be chosen and is fixed
2. The other datasets must be numeric

I think those are the only restrictions.
The real question is the UI. I'd propose adding a pane for widget columns in the view builder, right underneath the fixed columns. Any thoughts?