double t=1000;
double a=0.1*t, b=0.9*t;
...
TF1 *ffit = new TF1("ffit",fit,a,b,2);
ffit->FixParameter(0,t);
ffit->FixParameter(1,1);
...
TGraph *g = new TGraph(n,x,y);
g->Fit("ffit","R");