Changes 6.0:

1) Qt3 support dropped
2) QwtPlot layout/render code ported from int to double
  Exported/printed documents in scalable formats like SVG or PDF
  are 100% scalable now.
3) Template base classes introduced for curve and curve data
  to be reusable in all plot items displaying series of samples.
4) New plot items
  - QwtPlotHistogram
  - QwtPlotIntervalCurve ( error bars or displaying the area between 2 curves )
  - QwtPlotSpectroCurve ( mapping the z value to a color )
5) Raster items
  - QwtMatrixRasterData introduced
  - More accurate rendering
  - Several API changes
  - Thread support for rendering spectrograms
6) QwtPlot::print moved to QwtPlotRenderer
7) Other new classes
  QwtColumnSymbol
  QwtDoublePoint3D
  QwtIntervalSymbol
  QwtPlotDirectPainter
  QwtSamplingThread
  QwtSystemClock
8) QwtPicker and friends reorganized,
  QwtPickerTrackerMachine added for displaying a rubberband for
  mouse moves. Enter/Leave added to events, that are handled
  by the picker machines.
9) QwtScaleWidget::LayoutFlag added
  Introduced to control the direction of vertical axis titles.
10)QwtWeedingCurveFitter added
  QwtWeedingCurveFitter is an implementation of the Douglas/Peuker
  algorithm, that can be used to reduce the number of curve points.
  It can be very useful to improve the performance of painting
  curves of many lines ( f.e. by implementing different level of details ).
11)Legend code update for representing different pixmaps for different
  types of plot items.
12)Copy operators removed, using pointers instead
13)QwtPolarPoint from qwtpolar added
14)QwtThermo
  Optional QwtColorMaps added
15)Interfaces and code of all sliders/dials cleaned up.
  QApplication::globalStrut(), styled backgrounds ...

(adam)