Cannot fit graph

[Wile_E_Coyote]

It seems that we are talking about different macros …
I tried the last one that you posted, i.e. the “intensity”.
You seem to be talking about the first one that you posted, i.e. “Calibration”. In this macro, use:

graph->Fit(fit, "WQN"); // "initial pre-fit"
graph->Fit(fit); // "final fit"

Problems with errors in FIT

In ROOT, before you try to fit your graph or histogram, you MUST set “reasonable” initial values for ALL parameters of your function (except for some “built-in” formulas, like “gaus”, for which the standard fit procedure can automatically “guess” them), otherwise the fitting procedure may easily misbehave. So, try something like: f->SetParameters(-3000., 300.); See also the “initial pre-fit” + “final fit” trick in: