finding polynomial from interpolating between 2 other polynomial

Hi,
Have a look at the attached image. This is an aircraft performance chart. When we use these charts, we plot differents parameters (altitude, temperature, etc..) and, with a pencil, follow these curves and determine the value we are seeking for (fuel, time, distance, etc.)
I would like to acheive the same programmatically so I determined the polymnomials from the kown curves. However, there can be an infinite number of curves and when the plot falls between 2, I must find a way to come up with a new equation based on the interpolation of 2 polynomials.
The attached image shows a dotted line down to a reference line, then the user follow the closest curve (red) down to the know Y axis value, then vertically down (orange) to the value we are seeking.
What would you suggest?

Cross -posted here.

Ah - Chase-around charts... Many years ago I did this for B727 &B737 t/o performance data - very similar to what you're doing. I created a formula (mostly using natural logarithmic functions) to plot the values with given parameters then created another program to determine the variance from manual test plots. Not sure what you need but I needed FAA certification. I did this in C but excel can certainly handle it and the plotting as well. Not sure if this helps but allows me to reminisce..

If this approach looks feasible I'd be happy to expand on it.