Plot minimum of two arrays

Question

I have two arrays for x-values and two corresponding arrays for y-values which I wish to plot.

import numpy as np
import matplotlib.pyplot as plt

x1 = np.linspace(-2,2,100)
x2 = np.linspace(0,4,100)

y1 = x1**2+1
y2 = (x2-1.5)**2

plt.plot(x1,y1)
plt.plot(x2,y2)
plt.show()

This produces the following plot.

But instead of this, I want to plot only the minima of these two curves, i.e. only the region of y1 where y1<y2 and only the region of y2 where y2<y1. Something like this.

Since x1 and x2 are different, I can't use np.minimum(). Is there an efficient way to do this with numpy and/or matplotlib?

I would like to have a general approach that also works when y1 and y2 are not determined from some function I know, but are taken from e.g. a dataset.

So you want some kind of piece-wise function e.g y1 for x<0.5 else y2? You write that you want to plot the minima of both functions but it is not what you are plotting ? — CutePoison
– CutePoison, Commented Nov 16, 2021 at 11:43

JohanC · Accepted Answer · 2021-11-16 11:48:28Z

4

You could interpolate both functions onto a common x, and then take their minimum.

import numpy as np
import matplotlib.pyplot as plt

x1 = np.linspace(-2, 2, 100)
x2 = np.linspace(0, 4, 100)

y1 = x1 ** 2 + 1
y2 = (x2 - 1.5) ** 2

plt.plot(x1, y1, ls=':')
plt.plot(x2, y2, ls=':')
xc = np.sort(np.concatenate([x1, x2]))
y1c = np.interp(xc, x1, y1, left=y2.max(), right=y2.max())
y2c = np.interp(xc, x2, y2, left=y1.max(), right=y1.max())

plt.plot(xc, np.minimum(y1c, y2c), lw=10, alpha=0.4)
plt.show()

answered Nov 16, 2021 at 11:48

JohanC

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Comments

mozway · Accepted Answer · 2021-11-16 11:46:54Z

1

One solution is to cross compute the functions with the other xs and mask:

import numpy as np
import matplotlib.pyplot as plt

x1 = np.linspace(-2,2,100)
x2 = np.linspace(0,4,100)

y1 = x1**2+1
y1b = x2**2+1

y2 = (x2-1.5)**2
y2b = (x1-1.5)**2

plt.plot(x1, np.where(y1<y2b, y1, np.nan))
plt.plot(x2, np.where(y2<y1b, y2, np.nan))
plt.show()

output:

answered Nov 16, 2021 at 11:46

mozway

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2 Comments

The One Over a year ago

This works for this specific case, but I also want a solution that is more general; one that works when I don't define y1 and y2 myself through a known function, but extract them from a dataset instead.

mozway Over a year ago

@TheOne I see, you should have mentioned it, I'd have proposed a different approach ;)

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