QWT axis force major tick at zero - c++

Question

For a better unserstanding what I've done and what I've want to achive:
I want to set up a QwtPlot with a time Axis to display a signal (Voltage over time). I wrote a own implementation for the QwtScaleDraw so i can add an offset (lets say 1000µs) and change the "zero" value of the time axis.
No my system measures a event after 123µs plus this offset. So i want to display only this part of the plot with setInterval(1000, 1123) . But for better unserstanding of the plot I want to add some time before and after this event. Lets say 10% of the event length
10% of 123µs is 12.3µs => setInterval(987.7, 1135.3)
The axis show now a timespan from -12.3µs to 135.3µs.
NOW THE QUESTION: How could I force the axis to show a major tick at 0µs?
Do I have to use the QwtScaleEngine to calculate my ticks or is it a job for the QwtScaleDraw or is there already a function inside the Axis I've only missed?
EDIT:
I think the problem is my Offset. I substract this value direct from the original value inside the Function QwtText label(qreal value) const of my own implementation of QwtScaleDraw. So the zero to display is in this example the value 1000. Maybe its better so set the offset a other way?

Answer 1

I found a solution:
Create a own class inherit QwtLinearScaleEngine. Add a new member variable offset and add getter/setter. Overwrite the function divideScale and test if the given interval contains your offset. If not or when offset is null, use normal function of QwtLinearScaleEngine to calculate the divide scale.
When the interval contains your offset, go from that offset back (substract) with the given step size to the lower bound. Now you can go from this value up to the upper bound and add every value as a tick.
QwtScaleDiv QwtLinearScaleEngineEx::divideScale(double x1, double x2, int maxMajorSteps, int maxMinorSteps, double stepSize) const
{
QwtScaleDiv div = QwtLinearScaleEngine::divideScale(x1, x2, maxMajorSteps, maxMinorSteps, stepSize);
QVector<QwtScaleDiv::TickType> tickTypes = QVector<QwtScaleDiv::TickType>() << QwtScaleDiv::MajorTick << QwtScaleDiv::MediumTick << QwtScaleDiv::MinorTick;
QList<double> ticksByType[QwtScaleDiv::NTickTypes];
if(offset() == 0 || !div.contains(offset())){
for(auto type : tickTypes){
ticksByType[type] = div.ticks(type);
}
}else{
for(auto type : tickTypes){
auto ticks = div.ticks(type);
if(ticks.length() > 1){
stepSize = qAbs(ticks[1]-ticks[0]);
for( double i=offset()-(stepSize*floor((offset()-div.lowerBound())/stepSize));
i<div.upperBound();
i+=stepSize )
{
ticksByType[type].append(i);
}
}
}
}
return QwtScaleDiv(QwtInterval(x1, x2), ticksByType);
}