You can use more that one dimension in a combo chart, but then you can't use multiple expressions. A combo chart is a built-in object. You cannot extend it. It is possible to create an extension that will also show a combo chart and the does allow multiple dimensions with multiple expressions.
Though I have no idea how such a combination is supposed to give any meaning visible result in a single chart. One dimension on the x-axis and one dimension on a secondary x-axis (on top)?? Or perhaps nested dimensions like in this discussion: Line chart with two dimensions at x-axis issue....
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