Hi Franck,
sorry, I don't see your point.
I understand you use a base2 log instead of base10 log I did use, to rescale the x-axis.
It is a smart choice for legibility, as each interval lands into a doubling factor. But this would change nothing but the formula for the interpolation line.
Here is a graph including:
- blue squares and interpolation line for the elo points we have so far (as I did for my very first graph, but including updated games results we got since)
- orange line, the formula you suggest to approximate the elo according to log2(nodes): y=111,31x+311,31

The formula you suggest doesn't match the values we would like to approximate.
Or did I miss something?

The blue line formula is: y=171,4x - 184,49
So using your idea, the complete formula would be
elo=171,4 * log2(nodes) - 184,49

resulting in:
But of course, this would not scale well up to 1M nodes (would result in elo 3230+ ...)
Adding a theoretical 2400 elo point for 1M nodes, and drawing an updated line on this basis, would result in a further distant line from the above blue squares.
Thanks for your help,
Eric