Customizing QwtPlot's scale and grid lines - c++

Question

I'm confused with using QwtPlot class to draw a customized coordinate system.
My goal is to draw a coordinate system with customized scale, and draw grid line on these customized scales. Here are the values of the scales:
x-axis and y-axis scale:
x1=-0.642455
x2=0
x3=0.642455
y1=-0.642455
y2=0
y3=0.642455
I read the user's guide of QwtPlot class and QwtPlotGrid class. I found a function in QwtPlotGrid class called setxDiv and seemed that it could help, So I wrote the following code:
QList<double> doubleListmin;//min scale
QList<double> doubleListmed;//medium scale
QList<double> doubleListmaj;//major scale
doubleListmin.append(0.1);
doubleListmed.append(0.3);
doubleListmaj.append(0.642455);
QList<double> doubleList[3];
doubleList[0] = doubleListmin;
doubleList[1] = doubleListmed;
doubleList[2] = doubleListmaj;
QwtScaleDiv *xDiv = new QwtScaleDiv(-0.642455, 0.642455, doubleList);
//lowerbound is -0.642455, upperbound is 0.642455, doubleList customizes the scale
QwtPlotGrid *grid = new QwtPlotGrid();
grid->setXDiv(*xDiv);
//grid->updateScaleDiv(*xDiv, *xDiv);
grid->attach(this);
But it turned out to have no influence on the new QwtPlotGrid, it doesn't change its scale system.
I think there's another way: draw a several lines in the QwtPlot. But I don't know how to do it.
Somebody help me please!!!
Thank you in advance~

Answer 1

As you said "draw severalk lines" is not good approach because you can't do this lines infinity. To do this there is special class QwtPlotMarker which is more suitable here. Maybe not the best thing in the world, but works as you want:
QwtPlotMarker *m1=new QwtPlotMarker;
m1->setLinePen(QPen(Qt::gray));
m1->setLineStyle(QwtPlotMarker::VLine);
m1->setValue(0,0);
m1->attach(ui->qwtPlot);
QwtPlotMarker *m2=new QwtPlotMarker;
m2->setLinePen(QPen(Qt::gray));
m2->setLineStyle(QwtPlotMarker::HLine);
m2->setValue(0,0);
m2->attach(ui->qwtPlot);
QwtPlotMarker *m3=new QwtPlotMarker;
m3->setLinePen(QPen(Qt::gray));
m3->setLineStyle(QwtPlotMarker::HLine);
m3->setValue(0,-0.642455);
m3->attach(ui->qwtPlot);
QwtPlotMarker *m4=new QwtPlotMarker;
m4->setLinePen(QPen(Qt::gray));
m4->setLineStyle(QwtPlotMarker::HLine);
m4->setValue(0,0.642455);
m4->attach(ui->qwtPlot);
QwtPlotMarker *m5=new QwtPlotMarker;
m5->setLinePen(QPen(Qt::gray));
m5->setLineStyle(QwtPlotMarker::VLine);
m5->setValue(-0.642455,0);
m5->attach(ui->qwtPlot);
QwtPlotMarker *m6=new QwtPlotMarker;
m6->setLinePen(QPen(Qt::gray));
m6->setLineStyle(QwtPlotMarker::VLine);
m6->setValue(0.642455,0);
m6->attach(ui->qwtPlot);

Answer 2

Better create your own QwtScaleDiv object setting the ticks manually and assign them using QwtPlot::setAxisScaleDiv(). Update of the grid will happen automatically ( as long as you didn't decouple it explicitly ).
When you also need to manage the tick positions when zooming you have to derive your own scale engine instead - overloading the QwtScaleEngine::divideScale() method.