这个类可以理解为图层。通过setZ（）来设置图层深度。 同一深度只设置一个。
1、 void QwtPlotCurve::setRawSamples (const double * xData, const double * yData, int size )
Initialize the data by pointing to memory blocks which are not managed by QwtPlotCurve. setRawSamples is provided for efficiency. It is important to keep the pointers during the lifetime of the underlying QwtCPointerData class.
2、void QwtPlotCurve::setSamples (const double * xData, const double * yData, int size )
Set data by copying x- and y-values from specified memory blocks.
这俩函数区别：setRawSamples 直接引用内存中的数据，而setSamples则会拷贝。
默认值：
1、 style( QwtPlotCurve::Lines ),
2、 baseline( 0.0 ),
3、symbol( NULL ),
4、 attributes( 0 ),
5、paintAttributes( QwtPlotCurve::ClipPolygons ),
enum PaintAttribute
{
/*!
Clip polygons before painting them. In situations, where points
are far outside the visible area (f.e when zooming deep) this
might be a substantial improvement for the painting performance
*/
ClipPolygons = 0x01,/*!
Paint the symbol to a QPixmap and paint the pixmap
instead rendering the symbol for each point. The flag has
no effect, when the curve is not painted to the canvas
( f.e when exporting the plot to a PDF document ).
*/
CacheSymbols = 0x02
};
6、legendAttributes( 0 )
enum LegendAttribute
{
/*!
QwtPlotCurve tries to find a color representing the curve
and paints a rectangle with it.
*/
LegendNoAttribute = 0x00,/*!
If the style() is not QwtPlotCurve::NoCurve a line
is painted with the curve pen().
*/
LegendShowLine = 0x01,/*!
If the curve has a valid symbol it is painted.
*/
LegendShowSymbol = 0x02,/*!
If the curve has a brush a rectangle filled with the
curve brush() is painted.
*/
LegendShowBrush = 0x04
};