Matplotlib/pyplot: easy way for conditional formatting of linestyle?

Question

Let's say I want to plot two solid lines that cross each other and I want to plot line2 dashed only if it is above line 1. The lines are on the same x-grid. What is the best/simplest way to achieve this? I could split the data of line2 into two corresponding arrays before I plot, but I was wondering if there is a more direct way with some kind of conditional linestyle formatting?

Minimal Example:

import numpy as np
import matplotlib.pyplot as plt

x  = np.arange(0,5,0.1)
y1 = 24-5*x
y2 = x**2

plt.plot(x,y1)
plt.plot(x,y2)#dashed if y2 > y1?!
plt.show()

There were related questions for more complex scenarios, but I am looking for the easiest solution for this standard case. Is there a way to do this directly inside plt.plot()?

maybe you can find where the lines intersect, split the line you want to dash at that index, and then use plt.plot(x, y, linestyle="dashed") — Filip
– Filip, Commented Oct 21, 2020 at 0:46
Yes, this is my work-around at the moment. However, I am wondering if there is a simpler or more direct way to achieve this. — Tilman Hartwig
– Tilman Hartwig, Commented Oct 21, 2020 at 0:50

Sameeresque · Accepted Answer · 2020-10-21 00:53:24Z

3

You could try something like this:

import numpy as np
import matplotlib.pyplot as plt

x  = np.arange(0,5,0.1)
y1 = 24-5*x
y2 = x**2

xs2=x[y2>y1]
xs1=x[y2<=y1]
plt.plot(x,y1)
plt.plot(xs1,y2[y2<=y1])
plt.plot(xs2,y2[y2>y1],'--')#dashed if y2 > y1?!
plt.show()

answered Oct 21, 2020 at 0:53

Sameeresque

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Tilman Hartwig Over a year ago

Thanks a lot! This is a neat, pragmatic solution. However, it still requires some "pre-processing" of the data. So I think this also answers the second part of my question: there is no simple way to have conditional linestyle formatting in just one command(?).

Filip · Accepted Answer · 2020-10-21 01:00:59Z

@Sameeresque solved it nicely.

This was my take:

import numpy as np
import matplotlib.pyplot as plt

def intersection(list_1, list_2):
    shortest = list_1 if len(list_1) < len(list_2) else list_2
    indexes = []
    for i in range(len(shortest)):
        if list_1[i] == list_2[i]:
            indexes.append(i)
    return indexes


plt.style.use("fivethirtyeight")

x  = np.arange(0, 5, 0.1)
y1 = 24 - 5*x
y2 = x**2
intersection_point = intersection(y1, y2)[0]  # In your case they only intersect once


plt.plot(x, y1)

x_1 = x[:intersection_point+1]
x_2 = x[intersection_point:]
y2_1 = y2[:intersection_point+1]
y2_2 = y2[intersection_point:]

plt.plot(x_1, y2_1)
plt.plot(x_2, y2_2, linestyle="dashed")

plt.show()

Same princile as @Sammeresque, but I think his solution is simpler.

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