histogram used to draw the `TGraph` axis was created with errors and therefore
the histogram painter painted it with errors which was a non sense in that
particular case. This is now fixed. It was discussed
here: https://root-forum.cern.ch/t/horizontal-line-at-0-on-y-axis/30244/26.

Use Read/Write lock where relevant.

Cache the last result of TProcessID::IsValid and TProcessID::GetProcessWithUID as
most often the same PID will be used for most of the process lifetime.

This addresses: https://root-forum.cern.ch/t/copying-trefs-and-accessing-tref-data-from-multiple-threads/29417

With the automatic coloring options (PMC, PLC etc ...) it was easy to forget to add a drawing option.

This is now fixed. The issue was mentioned  here: https://sft.its.cern.ch/jira/browse/ROOT-9552

In TGraph2D::GetHistogram when the points were all in the same plane (along X or Y) at a
negative coordinate, the computed axis limits were not correct. This was reported
here: https://root-forum.cern.ch/t/potential-bug-in-tgraph2d/29700/5.

`TPaveText` containing the title was not correct.