[Python] Intersection of two data sets, one linear and one not

I need to find the intersection of a straight line and a stress-strain curve to determine the offset yield strength. I've been trying to use the following which successfully prints the two lines but places the intersection point in the wrong place. strain1 and stress1 are both rather large arrays extracted from excel and m and c are the corresponding values for the appropriate straight line for strain1 and stress1. Pls help this struggling physics student

strain_offset = strain1 + 0.002
stress_offset = m[0]*strain1 + c[0]
yield_strength_index = np.argwhere(np.diff(np.sign(stress1 - stress_offset)))[0][0]

def intersection_point(Ax1, Ay1, Ax2, Ay2, Bx1, By1, Bx2, By2):
d = (By2-By1)*(Ax2-Ax1)-(Bx2-Bx1)*(Ay2-Ay1)
if d:
uA = ((Bx2-Bx1)*(Ay1-By1)-(By2-By1)*(Ax1-Bx1))/d
uB = ((Ax2-Ax1)*(Ay1-By1)-(Ay2-Ay1)*(Ax1-Bx1))/d
else:
return None

x_intersection = Ax1 + uA * (Ax2 - Ax1)
y_intersection = Ay1 + uA * (Ay2 - Ay1)

return x_intersection, y_intersection


first = yield_strength_index
second = first + 1
# A points from the stress strain curve
Ax1 = strain1[first]
Ay1 = stress1[first]
Ax2 = strain1[second]
Ay2 = stress1[second]
# B points from the offset line
Bx1 = strain_offset[first]
By1 = stress_offset[first]
Bx2 = strain_offset[second]
By2 = stress_offset[second]

# run our function that finds the intersection point
x_intersection, y_intersection = intersection_point(Ax1,Ay1,Ax2,Ay2,Bx1,By1,Bx2,By2)

print(x_intersection,y_intersection)

fig,ax = plt.subplots()


ax.plot(strain1,stress1)
ax.plot(strain_offset,stress_offset)
ax.plot(x_intersection,y_intersection,'go')

ax.set_ylim([0,50000000])
ax.set_xlim([0,0.035])

plt.show()


It outputs the following graph enter image description here

Continue reading...