CODEWITHSUNDEEP
pythonmatplotlib
Bach
Bach

Reputation: 6217

Getting y value of a curve given an x value

Given a Line2D object, which is the output of pyplot's plot function, I wish to determine the y coordinate of a given x. For example, if

import numpy as np
import matplotlib.pyplot as plt

x = np.array([1.0, 4.0, 9.0, 16.0])
y = np.array([1.0, 2.0, 3.0, 4.0])
line = plt.plot(x, y)[0]

then

def get_y(line, x):
    ...

## This should print something close to 2.0
print get_y(line, 2) ** 2

I tried using path = line.get_path(), and then interpolate but things haven't worked as I have expected. I thought there must be a standard way of doing that...

Upvotes: 1

Views: 3801

Answers (2)

arn
arn

Reputation: 1

Maybe numpy.interp(x, xp, fp, left=None, right=None) can help you http://docs.scipy.org/doc/numpy/reference/generated/numpy.interp.html

Upvotes: 0

Sebastian
Sebastian

Reputation: 21

If i understand correctly you want to know the height y of a point p on a line ab.

Assuming that the line is infinite we can do it this way.

Calculate the line slope:

height = lineEndY - lineStartY #calculate the line "height"
lenghtX = abs(lineStartX - lineEndX) #calculate the length of line along the X axis
slope = height / lengthX;              #calculate the "slope" by deviding length of x by height
result = x * slope;                 #calculate the x approx height on the line

Upvotes: 1

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