title: How to clear QwtPlot curves before replotting?
body: <p>In the earlier versions of <code>Qwt</code> we add <code>QwtPlot::clear()</code> but now, I can't find it. </p>
  
  <p>Any ideas? I have a real time plot, so when replotting, the info before xmin is not really important and with time it crashes as the replot is replotting all the information before xmin.</p>