Plotting a semilog chart with Qwt

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();
}