We need to improve the creation of curves in the plot data dialog. At the moment the implemented logic is bound to one single x-column only and we also completely ignore columns having one of the error plot designations.

S. a. https://bugs.kde.org/show_bug.cgi?id=386787. From the comment there, the current logic won't give the user the desired results if we have something like

x y x y y y x y y_err

and similar constellations. If the user wants to plot here everything, we'll have problems. We need to refactor this part in LabPlot and to also decide what to do with the comboboxes we show now in the Plot Data dialog - the logic behind these comboboxes always assumes there is only one x-column and all the other columns are y-columns...