can toggle x and y for percentage table

I wasn't quite sure how to name this.  My usage was with groups of series, so I thought groups_as_rows, but maybe you can think of a better name.  invert_x_y came to mind, but it seems like it could potentially be inconsistent with the other tables, since those already seem to list the groups down the y.

Also, I wasn't sure if this made sense to fiddle with in the toJSON function in the model, so I chose not to. See:
js/models/dashboard-items/percentage_table.js