QwtScaleMap

QwtScaleMap  （包含  QwtScaleTransformation ）

A scale map.

QwtScaleMap offers transformations from the coordinate system of a scale into the linear coordinate system of a paint device and vice versa.

缺省情况下：

The scale and paint device intervals are both set to [0,1].   大小一比一的关系。类似于场景和视口的关系。

paint device interval 指的是窗口的坐标。

scale device interval指的是窗口映射到场景的坐标。

xMap[i].setScaleInterval( -1.5, 1.5 );
yMap[i].setScaleInterval( 0.0, 6.28 );

xMap[i].setPaintInterval( cr.left(), cr.right() );
yMap[i].setPaintInterval( cr.top(), cr.bottom() );

curve[i].draw( &painter, xMap[i], yMap[i], cr );