QCustomPlot中通过鼠标滚轮缩放图形使yAxis范围总是从零开始

在QCustomPlot第三方库的源文件中找到了qcustomplot.cpp中的 wheelEvent滚轮事件函数。其中原文档说明如下：

/* \internal
Event handler for mouse wheel events. If rangeZoom is Qt::Horizontal, Qt::Vertical or both, the ranges of the axes defined as rangeZoomHorzAxis and rangeZoomVertAxis are scaled. The center of the scaling operation is the current cursor position inside the axis rect. The scaling factor is dependent on the mouse wheel delta (which direction the wheel was rotated) to provide a natural zooming feel. The Strength of the zoom can be controlled via \ref setRangeZoomFactor.
  Note, that event->delta() is usually +/-120 for single rotation steps. However, if the mouse wheel is turned rapidly, many steps may bunch up to one event, so the event->delta() may then be multiples of 120. This is taken into account here, by calculating \a wheelSteps and using it as exponent of the range zoom factor. This takes care of the wheel direction automatical