I assume you actually mean inverting the entire graph? The "Bottom to Top" simply refers to the direction of the gradient fill (i.e. the "Area color (start)" is used at the base and it gradually moves to "Area color (end)" as it moves up the filled area).
I think you're asking for, for example, the "0%" point on the Y-Axis to be at the top of the scale and the "100%" point to be at the bottom? If so, then yes, there's a definite possibility of that. If I do add it though I'm not sure it would help with graphing the cores as it stands as you can't pick out individual cores and the "all cores" graph could be plotting any number of lines (for example my PC has 4 cores, and I know AMB do a tri-core processor), so it might be strange to plot some lines one way and others another.
I suppose I could add a facility to graph individual cores. Then you could layer them one on top of the other which would work as long as you turned off auto scaling... but then auto scaling makes the graphs a lot more readable/useful (IMO). What do you think would be best approach?
Anyway, I can definitely make the graphs render "upside-down"; I'm just not too sure how useful that will be on it's own.