A Polar Chart are a particular type of Parametric Chart.
Even if the Polar Chart isn't included directly into QCustomPlot, you can create one of this with a mathematical trick.
r(t) = (5*t-2)^2 --- Polar Function to represent
You can convert this function in a Parametric function in this way:
X(t) = r(t)*COS(t)
Y(t) = r(t)*SIN(t)
X(t) = (5*t-2)^2*COS(t)
Y(t) = (5*t-2)^2*SIN(t)
With this parametric equations you can use the Parametric Plot of the QCustomPlot library to build your own Polar chart without the direct use of a "Polar Chart Function".