void plot_mem_stat(const char* filename) {

  TFile* f;

  f = (TFile*) gROOT->GetListOfFiles()->FindObject(filename);
  if (! f) {
    f = new TFile (filename);
  }

  TTree* tree = (TTree*) f->Get("memuse");

  TCanvas* c1 = new TCanvas("c_mem_use","Memory usage statistics",0,0,600,400);
  tree->Draw("fVSize:fEvent");
}