Plotting a semilog chart with Qwt - c++

Question

I would like to plot a semilog chart using Qwt. I have not knowledge about Qwt but I was looking for some examples to guide my code. The problem is that at the moment I don't find someone. Could you help me with a simple code? I want to use a matrix where I can get the x-axis and y-axis values and use them to create the plot. Thank you!

Answer 1

Try this:
QwtPlot *myPlot = new QwtPlot;
QwtPlotCurve *curve1 = new QwtPlotCurve;
 
QwtPointSeriesData* myData = new QwtPointSeriesData;
 
QVector<QPointF>* samples = new QVector<QPointF>;
samples->push_back(QPointF(1.0,1.0));
samples->push_back(QPointF(2.0,2.0));
samples->push_back(QPointF(3.0,3.0));
samples->push_back(QPointF(4.0,5.0));
myData->setSamples(*samples);
curve1->setData(myData);
 
curve1->attach(myPlot);
I used here QVector but qwtplotcurve support double arrays and other things, but I like work with containers. You can choose the best for you. QPoint contains x and y values.
Qwt also offer logarithmic scale engine: http://qwt.sourceforge.net/class_qwt_log_scale_engine.html
I should to say that maybe something wrong with your Qwt, but next code works perfectly on my computer:
#include "mainwindow.h"
#include <QApplication>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QwtPlot *myPlot = new QwtPlot;
QwtPlotCurve *curve1 = new QwtPlotCurve;
QwtPointSeriesData* myData = new QwtPointSeriesData;
QVector<QPointF>* samples = new QVector<QPointF>;
samples->push_back(QPointF(1.0,1.0));
samples->push_back(QPointF(2.0,2.0));
samples->push_back(QPointF(3.0,3.0));
samples->push_back(QPointF(4.0,5.0));
myData->setSamples(*samples);
curve1->setData(myData);
curve1->attach(myPlot);
myPlot->show();
// MainWindow w;
// w.show();
return a.exec();
}