Thank you DerManu!
Yes, you are right about my mistake....but I still don't know how to fix the problem.
I can't change the interval that calls the data update slot.. I take the value from a UDP datagram (one datagram per minute), so I call the data update when the datagram is available..
If I don't change nothing in the code (original dividing by 1000 instead of 60000), the values are corretly added, but the "range" of my plotting is too small..for example with one value per minute I see just one value in the plot! I would like to expand my range..and I would be able to see more values in the plot (for example twenty).
When I divide by 60000 instead of 1000, I see a good plot..many values in a single plot, but I have that problem with the label of the time value...