Monitor a histogram while running

ROOT
6

Yes, just take a look at this Read a tfile while writing with another process

A simple (untested) implementation would be:

//Program 1
void filler() {
   TFile* f = new TFile("test.root","RECREATE");
   TH1D* h = new TH1D("h", "h", 100, 0, 10);
   for ( int i = 0; i < 1000000; i++ ) {
      usleep(10000);
      h->Fill(gRandom->Gaus(5,1));
      if(i%1000==0)
      {
          h->Write("",TObject::kWriteDelete);
          f->SaveSelf();
      }
   }
   f->Close();
   delete f;
   return;
}

//Program 2 in another terminal
void readnow() {
    TFile* f = TFile::Open("test.root");
    new TCanvas();
    TH1D *h;  f->GetObject("h",h);
    h->Draw();
}