QCustomPlot Discussion and Comments

for example x*x+y*y = 1

like

QVector<double> X(N),Y(N);
for(int i=0;i<n;i++)
{
X = cos( i/N*2*pi);
Y = sin( i/N*2*pi);
}

(the problem is that not only one value of y compate with value of x)

Use QCPCurve, as it is written in the QCPGraph documentation :)