QwtScaleWidget 该widget主要用途用来设置scale。（A Widget which contains a scale.）包含color bar, scale ， title。
与scale设置有关的类：
1、QwtScaleDraw：（A class for drawing scales. ）
QwtScaleDraw can be used to draw linear or logarithmic scales. A scale has a position, an alignment and a length,
which can be specified . The labels can be rotated and aligned to the ticks usingsetLabelRotation() andsetLabelAlignment().
After a scale division has been specified as a
QwtScaleDiv object using
QwtAbstractScaleDraw::setScaleDiv(const QwtScaleDiv &s), the scale can be drawn with theQwtAbstractScaleDraw::draw()
member.
2、QwtScaleDiv ：（A class representing a scale division. ）
A scale division consists of its limits and 3 list of tick values qualified as major, medium and minor ticks.
In most cases scale divisions are calculated by a
QwtScaleEngine.
QwtPlot中设置scale：