How to solve equation from curve fitting?

This is probably really easy, however I am new in Matlab and cannot find a solution, so i have a graph and saved the coeffs in workspace after using basic fitting using spline so that it matches the curve, my question is i need to solve x for f(x)=Y, without guessing everytime how what x could be, how can i solve the equation so that i can find x easy and fast. Thanks for your time

[Walter Roberson]

If I understand correctly, you have a set of piecewise polynomials (cubic coefficients with breakpoints) derived from a spline fit, and for given Y values you want to find the x that would give that Y.

The difficulty with this is that there would generally be multiple solutions.

For example you can fit a cubic spline to several periods of sin(x). If you then ask to solve for Y = 0, you are going to generate multiple results.

Do you want all of the results implied by the spline structure?

[Michael Adam]

yes exactly with breakpoints, i dont thinke there are multiple solutions.

Answer 1

[Michael Adam]

Spline with breakpoints and the graph looks like this for 500, 750, 1000, 1500 bar