Querying Keyframe Value from Polypath

Hi, I'm trying to create simple script for creating bounce.
I manage to do it with single value inputs like Size, capturing the value of keyframe from BezierSpline object
But "Center" input using PolyPath instead,
anyone know how to querying the keyrames value, something like {(frame : value),}

I believe in spline graph it shows as "Displacement" instead of Center.