Unable to create qwtPlot object

[jtso8]

I am currently trying to create a scatter plot but, I'm unsure of what the problem is. I found this code online and I was using it as a starting point but, I can't get the program to build. This is the code that I am using.

#include <cmath>
#include <QApplication>
#include <qwt-qt4/qwt_plot.h>
 
int main(int argc, char **argv)
{
  QApplication a(argc, argv);
 
  QwtPlot plot;
  plot.setGeometry(0,0,640,400);
  plot.setAxisScale(QwtPlot::xBottom, 0.0,2.0 * M_PI);
  plot.setAxisScale(QwtPlot::yLeft,-1.0,1.0);
 
  QwtPlotCurve sine("Sine");
  std::vector<double> xs;
  std::vector<double> ys;
  for (double x = 0; x < 2.0 * M_PI; x+=(M_PI / 10.0))
  {
    xs.push_back(x);
    ys.push_back(std::sin(x));
  }
  sine.setData(&xs[0],&ys[0],xs.size());
  sine.attach(&plot);
 
  plot.showFullScreen();
  return a.exec();
}

#include <cmath>
#include <QApplication>
#include <qwt-qt4/qwt_plot.h>

int main(int argc, char **argv)
{
  QApplication a(argc, argv);

  QwtPlot plot;
  plot.setGeometry(0,0,640,400);
  plot.setAxisScale(QwtPlot::xBottom, 0.0,2.0 * M_PI);
  plot.setAxisScale(QwtPlot::yLeft,-1.0,1.0);

  QwtPlotCurve sine("Sine");
  std::vector<double> xs;
  std::vector<double> ys;
  for (double x = 0; x < 2.0 * M_PI; x+=(M_PI / 10.0))
  {
    xs.push_back(x);
    ys.push_back(std::sin(x));
  }
  sine.setData(&xs[0],&ys[0],xs.size());
  sine.attach(&plot);
  
  plot.showFullScreen();
  return a.exec();
}

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The errors that I get don't really tell me anything. The errors tells me the file path of these things and says undefined reference to:
QwtPlot::QwtPlot(QWidget*)
QwtPlot::setAxisScale(...)
QwtPlot::setAxisScale(...)
QwtPlot::~QwtPlot()
QwtPlot::~QwtPlot()

just to name a few. Does anyone know a possible solution to this problem.