Changing the number of Decimal places

Is its possible to change the number of decimal places shown
used when using the “text” option when plotting a 2D histogram ?

use gStyle->SetPaintTextFormat(format)


All details about the TEXT option are here:

Yes, that was the first thing I tried. The problem is that it doesnt work
when I write the resulting histogram to a file, and then read it afterwards.

Ok, send a small running macro showing the problem you encountered and we will help you.

Sorry, I understand what you mean now. This setting is something global and therefore not stored in the histogram data structure, so it is not persistent when you same the histogram. You should set it again before Drawing the histogram. It can be put in your rootlogon.C macro.