Hello, Riccardo;
 
In order to fix the problem it is necessary to replicate it. Have you been
using the snapshot version all along? If you install earlier versions of
gretl and/or gnuplot, does the problem persist?
 
Yes, I have been using the snapshot version and the problem appears with 1.6.5 version, but when I install earlier version of Gretl (1.6.0, specifically) the problem dissapears
 
 Does it happen only with
the particular series you sent in an earlier message, or does it happen
all the time?
 
It happens with the serie I sent and with other series, but not with the most of series
 
Cheers
Javi