Find element in 2d array closest to a specified value within a certain section of that array

Question

I am trying to calculate the gradient of the linear section at the beginning of a line graph that I have plotted from a 2D array. Instead of looking through the array myself to find the values that I need to calculate this gradient, I want code that will do this for me. For example, if it looks like the linear section ends at around 200MPa (The black line here)

I want the code to find the stress value and corresponding index that is nearest to 200 in the first section of the graph (first 2% on the x axis)

The code I have written so far finds the closest value but includes the whole array so gives points near the end of the curve occasionally

def find_nearest(array, value):
    diff = np.abs(array - value)
    idx = np.where(diff == diff.min())
    return idx


valueA = 200
print(find_nearest(TensA, valueA))