Question: How to find zero position/origin for Qtcharts graph

[RTjen]

Hello,

I am trying to find a function to get the position value of the zero position or origin in Qtcharts-Spline chart.
The red dot in this graph is the position I would like to find out.
http://i.imgur.com/ZLh59ft.png

using globalPos(), windowPos(), and pos() cannot seem to find the position of the origin.

Thank you.