How do I set proper zoom scale for each refresh

[rawfool]

I'm plotting a histogram(used from TV Plot example) on QwtPlot. I'm using a QwtPlotZoomer object and setting it's canvas to my QwtPlot. I have a plot button which plots the current values. When the data values are changed after sometime and if I click plot, the new data points are plotted.

Now the problem is after plotting for first time, when I zoom the plot area, it's working fine. But when I refresh the graph again, now if I zoom, it's zooming from the previous zoom level. How do I avoid this ?

[Spitfire]

Did you set correct zoom base after changing the data?