What I wish to do: Find the intersection of the fitted curve with another curve(which happens to be a straight line.
How:
1. Get Curve Points
QwtSplineCurveFitter *fit = static_cast<QwtSplineCurveFitter *>(orgnlCrv->curveFitter()); ///find curve Fitter
int div = fit->splineSize();//find spline size of curve fitter
QwtSplineCurveFitter fit2;//create a local curve fitter object
fit2.setSplineSize(div); //set spline size to that of orginal curve
fit2.setFitMode(QwtSplineCurveFitter::Spline);//specify type of fit mode

QPolygonF crvPt, d_pt; //d_pt has data points of original curve
populateD_pt(); //utility function which populates d_pt
crvPt= fit2.fitCurve(d_pt); //

2. Make a QList of QlineF with crvPt //crvPt[i] and crvPt{i+1], iterate i
3.Use intersect method of QLineF with each of segments of original curve with the second curve. Find the intersection pt.

Problem QPolygonF crvPt has integer values! The resulting intersection point is at integers on graph.

Right now I assume linear interpolation between two data points of original curve, but the intersection point I get with this assumption is not exact obviously! It would be great if i can get the exact intersection point with the original curve.