Is it possible to draw a graph on an existing graph, and choose the marker type?
Otherwise, I would like to draw a graph with two specific points of a different colours. is it possible to do that without using two different graphs (and therefore superimpose them later)?
The most reliable way to superimpose graphs is with a TMultiGraph. Make the TMultiGraph and ->Add the existing TGraphs to it. If you ->SetMarker* (Color, Style, etc) each TGraph, they will be drawn with their set marker styles.
Drawing with “same” is faster in principle, but to be honest I never found it satisfactory. The TMultiGraph method allows you more control of how the graphs are drawn w.r.t. each other.
Note: for TH*, the equivalent container is a THStack (which I always draw with the “nostack” option).