Qwt: log scale and ticks every 10th of a decade

I made a QwtPlot with an x log scale and a y linear scale. The x axis range is fixed from 10 to 1000, i.e. 3 decades. A added a grid with major and minor divisions.

The problem : the minor ticks fall at some automatically calculated positions.

What I want : a minor tick every 10th of a decade (20, 30, ..., 200, 300, ...).

I tried to play around, following various examples found here and there. But anything I tried didn't change a thing. I'm sure I missed something in Qwt's documentation...

Qwt version : 6.1.4

Solution

I figured it out. In a subclass of QwtPlot :

setAxisAutoScale(QwtPlot::xBottom, false);
xlogScale = new QwtLogScaleEngine();
QwtScaleDiv xDiv = xlogScale->divideScale(10.0, 1000.0, 2, 10, 1.0);
setAxisScaleEngine(QwtPlot::xBottom, xlogScale);
setAxisScaleDiv(QwtPlot::xBottom, xDiv);

The key is the 3rd line, which sets the x axis scale. For some reason, I had trouble to understand what arguments the function expects :

from 10 to 1000
2 major steps
10 minor steps per decade
major step size = 1 decade

And also, don't call QwtPlot::setAxisScale() ! It resets everything to default.

Hope it helps !