Thanks a lot! Its working now!
Another quick question!
When I load a bigger file (about 3,5 mins or bigger) the Qt crashes and I think it's because qwtplot replots so much. It has to plot about 6 million dots. Its Qt's fault or there's a solution? And when I want to get a playback position indicator cursor (a vertical line) for my plot, it replots so much the Qt can't do it. Can I somehow cache the plot and then do the cursor over only the "picture" of the plot?