Fix a precision issue when the min and max values are equal on TGraph2D axis.
Fix a precision issue when the min and max values are equal on TGraph2D axis.

This was reported here: https://root-forum.cern.ch/t/tgraph2d-plotting-issues/26562

zzxuanyuan/root,olifre/root,olifre/root,olifre/root,karies/root,root-mirror/root,karies/root,zzxuanyuan/root,karies/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,zzxuanyuan/root,zzxuanyuan/root,zzxuanyuan/root,zzxuanyuan/root,zzxuanyuan/root,karies/root,zzxuanyuan/root,root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,zzxuanyuan/root,root-mirror/root,root-mirror/root,zzxuanyuan/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,zzxuanyuan/root,olifre/root,zzxuanyuan/root,olifre/root,olifre/root