qwtplotgrid and qwtscalediv problem

[Uwe]

at x = 15, 30, 45, 60, 90, 105, 135, the vertical grid lines is the minor grid lines(gray, dashline)
at x = 0, 75, 150, the vertical grid lines is the major grid lines(black, soildline)

When using setAxisScaleDiv() the ticks will be exactly, where you have defined them. Of course another call of setAxisScale() or setAxisAutoScale() for the same axis will overwrite your definition.

does that mean I should overload QwtPlotGrid::draw()?

No all what needs to done is to define range and ticks in a QwtScaleDiv object and assign it to the scale. The grid will be synchronized to it in the next replot.

Uwe