qwt zoom out with wrong reect - qt

Question

I have a qwt plot, here a simple example, with a QWtPlotZoomer. The zoom in works, but zoom back reset the scale to 0-1000, instead the original scale.
I tried to solve this with (what is the difference between these lines ?)
zoomer->setZoomBase(false);
zoomer->zoom(0);
but that has no effect. What needs to be done to get the correct initial scale for the zoomer? A trivial solution is to implement the zoomer after the curve attachment, but in a real work example that is not applicable:
#include <QtWidgets/QApplication>
#include <QtWidgets/QMainWindow>
#include <QDebug>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_plot_zoomer.h>
#include <vector>
using std::vector;
int main( int argc, char **argv )
{
QApplication a( argc, argv );
QwtPlot * plot = new QwtPlot();
plot->setAxisAutoScale(QwtPlot::xBottom);
plot->setAxisAutoScale(QwtPlot::yLeft);
QwtPlotZoomer *zoomer;
zoomer = new QwtPlotZoomer( QwtPlot::xBottom, QwtPlot::yLeft, plot->canvas() );
// create data
vector<double> x(100);
vector<double> y1(x.size());
for (size_t i = 0; i< x.size(); ++i) { x[i] = int(i)-50; }
for (size_t i = 0; i< y1.size(); ++i) { y1[i] = pow(double(abs(i-50))/10,2); }
// first curve
QwtPlotCurve *curve = new QwtPlotCurve();
curve->setRawSamples(&x[0], &y1[0], x.size());
curve->attach( plot );
zoomer->setZoomBase(false);
zoomer->zoom(0);
plot->replot();
QMainWindow window;
window.setCentralWidget(plot);
window.resize(800, 600);
window.show();
return a.exec();
}

Answer 1

Create the zoomer after attaching the curves ( having valid data ) or modify your code to zoomer->setZoomBase(true);
The line zoomer->zoom(0) is pointless and the final replot is not necessary as it is done by the zoomer to initialize its zoom stack.
When having a zoom base of [0,1000] you usually have initialized the zoom stack of your zoomer with a plot, where the scales have not been calculated before.
Note that attaching the curves does not update the ranges immediately - it is done before the next replot ( or better QwtPlot::updateAxes() ).