Qwt spline fitter - it doesn't work as expected

[giaur]

Hello,

I need to use spline interpolation for data display in my program. There is QwtSplineCurveFitter class - documentation says, thar it uses spline to fit curve, but the results are not acceptable (or I have missed something?):

Code to fit curve between 5 points:

QwtPlotCurve *curve1 = new QwtPlotCurve("Curve 1");
 
QPolygonF polygon;
 
polygon << QPointF(-1, -3);
polygon << QPointF(1, 5);
polygon << QPointF(2, 16);
polygon << QPointF(3, 8);
polygon << QPointF(4, 13);
 
QwtSplineCurveFitter fitter;
fitter.setFitMode(fitter.ParametricSpline);
curve1->setData(fitter.fitCurve(polygon));
curve1->attach(ui->plot);
 
ui->plot->replot();

QwtPlotCurve *curve1 = new QwtPlotCurve("Curve 1");

QPolygonF polygon;

polygon << QPointF(-1, -3);
polygon << QPointF(1, 5);
polygon << QPointF(2, 16);
polygon << QPointF(3, 8);
polygon << QPointF(4, 13);

QwtSplineCurveFitter fitter;
fitter.setFitMode(fitter.ParametricSpline);
curve1->setData(fitter.fitCurve(polygon));
curve1->attach(ui->plot);

ui->plot->replot();

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And this is the result:



Without using fitter class:



OS: Debian Sid
Qwt: 5.2.0-1 (libqwt5-qt4)

What's wrong there? Interpolation seems to be not working at all or spline resolution is very low but I can't see any method to increase it?