If you have the plot I will assume that you have arrays of X and Y values. For each of the X values:
1. Use the Threshold 1D Array function with the array of X values as the array input and your target X value as the scalar input. The output is a fractional index of where that X value falls in the array of X values.
2. Use the Interpolate 1D Array function with the array of Y values as the array input and the fractional index from the threshold operation as the index input. The result is the interpolated Y value at the specified fractional index.
Mike...