use only TGraphs

Change to write only TGraphs instead of TMultiGraphs and TCanvases to enable superimposing plots, e.g. as in: https://lblhcbpr.cern.ch/#/app/rootfileviewer?apps=5&withNightly=false&checkSpecific=false&nightlyVersionNumber=10&jobs=61872&jobs=61883 FYI @dovombru @sstahl

unmarked as a Work In Progress

changed the description

@dovombru following our e-mail conversation, you can merge it when you'll verify that the output ROOT file is fine.

Closing due to issues with drawing.

closed