do you have the rpm range these cars were using to give the graphed results? Seeing as you have instantaneous power readings, it might be helpful to calculate the average power/rpm figure as this is far more important than peak. Obviously the BMW would have the highest average yet it could give better distinguished results between those cars who overlap each other a lot on the graph.
Other than that, your pre-existing graph is easy to read and a good comparison to go by.
