Make linear calibaration fitting

can you help in this code please !
the attached file.txt (can be readed by Notepad or by Atom contain a result of spectrometer, (Count).
the file only show the count. the channels range from 0:8192.
how can i coded the calibration process for energy.

#include "TGraph.h"
#include "TPad.h"
#include <string>
#include <fstream>
#include <cstdlib>
TGraph *mca(const char *file = "file.mca") { // "MCA8000D PMCA SPECTRUM"
  TGraph *g = new TGraph();
  g->SetTitle("MCA8000D PMCA SPECTRUM;channel number;number of counts");
  if ((!file) || (!file[0])) return g; // just a precaution
  std::string line;
  std::ifstream ifs(file, std::ifstream::in);
  while (std::getline(ifs, line) && (line.find("<<DATA>>") != 0)) continue; // wait for "<<DATA>>"
  while (std::getline(ifs, line)) {
    if (line.find("<<END>>") == 0) break; // wait for "<<END>>"
    double x = g->GetN(); // raw "channel number"
    // x = 10. * x + 100.; // here you can implement some energy calibration formula
    double y = atof(line.c_str()); // raw "number of counts"
    // y = y / 100.; // here you can implement some count rate scaling
    g->SetPoint(g->GetN(), x, y);
  if (g->GetN() > 0) g->Draw("APL");
  return g;
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