First off thanks for all your help beef, I combined both data series into one line chart with two Y axes, and didn't have to change the L/100 km to negative values as there was an option for 'values in reverse order'.
By adjusting the maximum/minimum values for each axis I can fine tune how the two curves line up with each other, but is this an accurate way to compare the data? Feels like I'm 'cheating' the results?
Doogals 
Threaded Mode
