How to add tick marks and text right above plot

Newbie
4

Perhaps I should clarify what I mean by tick marks. Let’s say I want to a tick mark on the top horizontal line of the plot at the x axis value (star age value) of 12.75403. Is there a way to specifically place tick marks on top of the plot at a point that correlates with an x value?

I attempted to do this with the modification, after reading here:

   c1->Update();
   c1->Range( 0., -500., 30., 500. );
   auto line = TLine(12.75403,0,12.75403,500);
   line.SetLineColor(kYellow);
   line.SetLineWidth(2);
   line.Draw();

But I am not getting a yellow line over my plot.