Example which shows how you can get the graph of the lower plot and set the y axis range for it. 
Since the lower plot is not created until TRatioPlot::Draw is called, you can only use the method afterwards.
  
void hist031_TRatioPlot_residual_fit()
{
   auto c1 = 
new TCanvas(
"c1", 
"fit residual simple");
 
   auto h1 = 
new TH1D(
"h1", 
"h1", 50, -5, 5);
 
   rp1->GetLowerRefGraph()->SetMinimum(-2);
 
   rp1->GetLowerRefGraph()->SetMaximum(2);
 
}
R__EXTERN TStyle * gStyle
 
1-D histogram with a double per channel (see TH1 documentation)
 
virtual void FillRandom(TF1 *f1, Int_t ntimes=5000, TRandom *rng=nullptr)
 
virtual TFitResultPtr Fit(const char *formula, Option_t *option="", Option_t *goption="", Double_t xmin=0, Double_t xmax=0)
Fit histogram with function fname.
 
virtual void SetMinimum(Double_t minimum=-1111)
 
virtual void SetTitle(const char *title="")
Set the title of the TNamed.
 
Class for displaying ratios, differences and fit residuals.
 
void SetOptStat(Int_t stat=1)
The type of information printed in the histogram statistics box can be selected via the parameter mod...
 
- Date
 - January 2020 
 
- Author
 - Paul Gessinger 
 
Definition in file hist031_TRatioPlot_residual_fit.C.