I'm not absolutely sure but I think you touched the max. amount of dimensions and expressions which are possible with a stacked combo-chart. I think your line isn't really a line against the dimensions else just a single value - if you changed the type from line to point it should be clearer.
An alternatively might be to concat the two dimensions (in datamodel or in the chart - just Dim1 & ' - ' & Dim2) or to enable a grid for one dimension (bottom left in tab dimension). Another way would be to use two specialized charts which are on to of eachother (with the apropriate layout-adjustments to the axis, border, title and so on).
- Marcus
