This dialog can be opened via the "File -> Import ASCII -> Set import options" menu.
Here you can specify the column separating characters (for instance ",") in the "Separator" list.
If the "Auto" option is selected, QtiPlot will try to guess which could be the column separator combination.
It's only a best effort guess and it might fail. In this case you might want to define the correct separator sequence.
If it doesn't figure in the the "Separator" list, you must select the option "Define" and click the "OK" button.
An input text dialog will pop-up, allowing you to type the separator.
The number of first rows to be ignored can also be customized here.