When you choose a file, you also choose which items should be the x and y axis on your plot. You will choose the items on the "Select items" page in the "Open" dialog.
After selecting the items, you can select line appearance or marker style.
The "Item pairs" box gives information on the items, like min/max values and units. The only thing you can choose here is whether the y axis should be on the left or right in the picture ("Axis pos" = 1 or 2 respectively).