如何在python中获取标绘函数值

Question

I have 2 discrete coordinates, X and Y. I call plt.plot(X,Y) and get nice plot. Not I want to get the values of X(the X is that is not included in the file) from that plot(function graphic). For example I have X=[1,2,3,4] Y=[0.4,0.5, 0.8, 0.85] how to get the value for X when Y = 0.43 ?

Answer 1

With numpy:

import numpy as np

X=[1,2,3,4]
Y=[0.4 ,0.5, 0.8, 0.85]

y = .43
x = np.interp(y, Y, X)

Without numpy:

X=[1,2,3,4]
Y=[0.4, 0.5, 0.8, 0.85]

y = .43

for i, yi in enumerate(Y):
    if y < yi:
        break

k = (X[i] - X[i-1])/(Y[i] - Y[i-1])
m = X[i-1]

x = k*(y-Y[i-1])+m

Note, that the function y = f(x) has to invertable (definite) and y has to be within the range of Y for this to work, as I haven't added any boundry checks.

What I do, is that I calculate the slope in the interesting region, and then use that to find the value at an offset from Y[i-1]