How to interpolate to get two values for one variable?

I have a graph which is ascending in the beginning and will descend afterwards.

Let's say my y-axis is Y, and x-axis is X,

for a certain Y value, it will give two values on the x-axis.

say in this case i set Y to a value of 20, but it only gives me one X value, which is the latter value when the graph is in its descending portion. How do I get the X value for the initial ascending part of the graph?

I did something like this: interp1(Y,X,20)

Kindly advice.. Thanks!

Answer 1

[John Petersen]

Use the same index for each parameter. If you want to know the values of X for a given value of Y,

yind = (Y==yvalue);  
xvalues = X(yind);  % values of x that correspond to yvalue