In ROOT, you are expected to provide reasonable initial values of all fit parameters (except for some “built-in” functions, like “gaus”, for which the standard fit procedure can automatically “guess” them). Get used to it.
Note: if you need to swap “X” and “Y” columns, you can try:
TGraph *gYX = new TGraph("ev7.dat");
TGraph *g = new TGraph(gYX->GetN(), gYX->GetY(), gYX->GetX());
delete gYX; // no longer needed