Paul, I think your graph is a power required curve, usually used to demonstrate the different sources of drag that must be overcome with power.
I'm not sure it is valid in that form for autorotation since your rate of descent at zero speed would have to be significantly higher than at bucket speed.
That graph should be drawn with a discontinuity on the left hand axis since the total drag curve is additive (ie the sum of the other 3 curves at any point on the horizontal axis).
The drag according to your graph for a zero speed auto would be probably twice that at bucket speed because the curve would be more pronounced.
That would mean you would need twice as high a RoD at zero speed compared to bucket speed. I haven't flown a helicopter yet where that is true.