Thank you very much!
I'm plotting a scatter diagram with a regression line. I just want to make the line fit the chart so I made a curve with two points taking the lower and upper bounds as the x coordinates.
Now it's solved when I get the range of xbottem after a replot and set the line and replot agian...
QwtPlotItem::ScaleInterest flag seems to make sense to me but I failed to find it.
QwtSeriesData::setRectOfInterest() may also do the trick when I understand it after some test.