Addition of steffen interpolation to fit functions?

[mlnorthernswe]

Maybe not an issue but I wonder if it's possible to add steffen interpolation to the fit functions?
It's a montonic spline function that I think suits the absolute measurements data. For example with that method local extrema can't occure between two knots (absolute measurements). Only at the knots which is preferred for magnetometer data. It's not so good if there is a jump or an oscillation between two knots (or two absolute measurements). With steffen spline the curve will preserve it's softness despite of that.
I'm using that for our data and get good G values between absolute measurements.
I see in gvar steffen spline is implemented in python.