How do I use two different sets of data for X axis when plotting graph?

I am TRYING to plot a graph for Economics with 'Output' on the X Axis (1,2,3...10) and $ (0,50,100...300) on the Y Axis. There are 3 Lines on the graph using this data (eg. Total fixed cost, Total variable cost, Average variable cost) but then I need to put a 4th line on the graph for Marginal Cost - using the same Y Axis values, but plotting on the X axis at 1.5,2.5,3.5...9.5. I don't have these values for the other 3 lines that I am plotting though so end up with a graph that is completely wrong. Is there anyway I can plot this 4th line without messing up the rest of the data?

Any help is much appreciated!!

Right click on line you want to have different axis.
-> format data series
-> choose in Plot series on -> Secondary axis

Oh, yes, that might be it... didn't read carefully thou