QChart determine the y coordinates by the x coordinate

[Sandres92]

How to determine the y coordinates by the x coordinate. The x coordinate is known but the y is unknown
I use QChart , QLineSeries
Maybe there are functions for determining one number (y) by another (x)?
Or is it possible to find the intersection of the two QLineSeries? (blue and black)

[VRonin]

It's not a QChart problem but a math one. QLineSeries dose nothing but interpolating linearly 2 points.
Take the closest 2 numbers in the series to x (the one just higher and the one just lower) and calculate the linear interpolation

double yForX(double targetX){
const QList<QPointF> linePoints = lineSeries.points();
if(linePoints.isEmpty()) return 0;
auto i=linePoints.begin();
if(i->x()<=targetX) return i->y(); // x < minimum
for(const auto iEnd=linePoints.end();i!=iEnd;++i){
auto j=i;
if(++j==iEnd) return i->y(); // x > maximum
if(j->x()<targetX) continue;
if(j->x()==targetX) return j->y();
return (targetX-i->x())*(j->y()-i->y())/(j->x()-i->x());
}
Q_UNREACHABLE();
}