title: Qwt - how to get the select point index with QwtPlotPicker
body: <p>I plot complex data using QwtPlot, so, I reimplement QwtSeriesData for converting my data into QPointF. The "sample(int)" converts MyObject to QPointf. My QwtSeriesData data manages a </p>
  
  <pre><code>myVector=vector&lt;MyObject&gt;;
  </code></pre>
  
  <p>Now, I need to select point drawn into my plot. I try to use QwtPlotPicker but it only returns a QPointF. </p>
  
  <p>How to get the index "idx" of the selected point in order to get <code>myObject=myVector[idx]</code>?</p>