getting rid of white space in graphic

[Wolfgang Engelmann]

Slightly off topic: Normally I can cut off white space of a graphic by 
using the limitation box (Auf Begrenzungsbox zuschneiden, in German). 
But this does not work in the appended figure. Same, if I convert it in 
jpg, png, pdf). What could be the cause and is there an alternative by 
using a special program (gimp works, but with many figures its a bit 
cumbersome).

Wolfgang