TGaxis with function

Dear ROOTers,

I make an attempt to draw a second axis for my TGraph, but it seems that I do not fully follow the idea of funciton in TGaxis constructor. The standard y axis of my TGraph is linear. The second axis should display values equal to -2.5*(log10(0.039)-7.98/2.5), where x are the values of the standard y axis. On the plot below lines show the values calculated according to this formula, and an axis attempt made according to this formula. Axis does not follow the lines, thus the formula…

I also attach the script used to make the plot.
axis_attempt.C (2.42 KB)

Really noone can provide me a clue, what am I doing wrong?

You should do:

TF1 *af = new TF1(“af”, “pow(10, -x/2.5) / pow(10, -7.98/2.5)”, 11, 12.5);