You should be able to do it this way:
1. First add both columns to same section of Y-axis (select them both at the same time and drag and drop to Y-axis and select the visualization you want.
2. Right click on graph and choose add to add the visualization you want (you can also shift click to select the visualization from the top of graph)
3. Then from the left of graph expand Variables menus and uncheck unnecessary columns
-Jarmo