Interpolating between lines

Hi all,

I am trying to digitize a performance graph for a single engine aircraft.

I have used webplotdigitzer and ploty to extract the x,y values and find the equation for the lines.

I am having trouble finding an accurate interpolation between the pressure altitudes. I've tried FORECAST.LINEAR and TREND but they are not accurate enough.

What would you recommend on doing?

Screenshot (308).png

Thanks.