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.
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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)
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3. Then from the left of graph expand Variables menus and uncheck unnecessary columns
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-Jarmo