The best solution should be to translate the x coordinates from pcm to milliseconds. As you probably don't want to copy your samples for this I recommend to derive from QwtSeriesData<QwtPointF> and do the translation there. F.e. something like this:

Qt Code: 
  1. class YourSeriesData: public QwtSeriesData<QwtPointF>
  2. {
  3.         virtual QPointF sample( size_t i ) const
  4.         {
  5.             // depends how you have stored your samples
  6.  
  7.             double xPCM = ...;
  8.             double y = ...;
  9.  
  10.             return QPointF( toMS( xPCM ), y );
  11.         }
  12. };
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Guess overloading YourSeriesData::boundingRect makes also sense as the x-range can be found easily for a series with increasing x values.

HTH,
Uwe

PS: It's always a good idea to check the cpuplot example - even if completely unrelated when you don't want to display the tick labels in a date/time format