Limit line painting on a graph

ROOT
2

You can plot a TLine after plotting the histogram, like this

float threshold = 43.69;
auto lnThreshold = new TLine(threshold, 0, threshold, gPad->GetUymax());
lnThreshold->SetLineColor(kRed);
lnThreshold->SetLineStyle(10);
lnThreshold->SetLineWidth(2);  // maybe
lnThreshold->Draw();

Since the y axis is logarithmic, have a look into this topic on how to correctly draw the line all the way to the top of the pad.

A TText object can be used to draw the word “threshold” in the plot.