Quoted from Circus_Animal:I assume there's no way of doing it, but a graph showing the combined average rating for all machines would provide some useful context for the above graphs.
I did something similar in 9/2013. Average then was about 7.3. I agree with all of the points regarding vote manipulation but this was a fun exercise for me.
http://toddnappi.blogspot.com/2013/09/an-analysis-of-pinside-pinball-rankings.html