Your code is not handling the values correctly because you are adding numeric values together by using concatenate
If you select first item in combobox2 then the listindex of zero has no affect, so you code appears to work. Select anything in combobox1, except first item, and any item except first from combobox1 and then the chartIndex value will be missed by select case.
When combining the plot should the 2 series be put on separate axes? If so is it combobox1 on left and combobox2 on right?
Do you always want 2 lines plotted or should it be possible to select nothing to plot?
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