Linewidth is added to the length of a line

Question

When I draw a line segment in matplotlib the linewidth seems to be added to the length of the line. Below my code (not the most pythonic code, but it should do the trick). Am I doing something wrong or is this just a feature of matplotlib?

import matplotlib.pyplot as plt
import numpy as np

L1 = 100
L2 = 75
L3 = 100
Y = 3
N = 5
l_prev = 0
for l, c in zip(np.linspace(0, L1, N), range(N)):
    plt.plot([l_prev, l], [0, 0], 'r', linewidth=20)
    l_prev = l
l_prev = L1
for l, c in zip(np.linspace(L1, L1 + L2, N), range(N)):
    plt.plot([l_prev, l], [Y, Y], 'g', linewidth=1)
    l_prev = l
l_prev = L1
for l, c in zip(np.linspace(L1, L1 + L3, N), range(N)):
    p = plt.plot([l_prev, l], [-Y, -Y], 'b', linewidth=10)
    l_prev = l
plt.axvspan(xmin=L1, xmax=L1)
plt.axis([-5, 205, -5, 5])
plt.show()

What I expected to see is three line segments: [0,L1], [L1,L2] and [L1,L3]. But the first line [0,L1] extends to L1 + 'the diameter'....

score 64 · Accepted Answer · edited Apr 28 '12 at 05:10

64

It looks like the default solid_capstyle is projecting, which isn't the one you want:

plt.figure()
plt.plot([0, 100], [5, 5], linewidth=50, linestyle="-", c="blue",
         solid_capstyle="butt")
plt.plot([0, 100], [15, 15], linewidth=50, linestyle="-", c="red",
         solid_capstyle="round")
plt.plot([0, 100], [25, 25], linewidth=50, linestyle="-", c="purple",
         solid_capstyle="projecting")
plt.axvline(x=100, c="black")
plt.xlim(0, 125)
plt.ylim(0, 30)
plt.savefig("cap.png")

enter image description here

edited Apr 28 '12 at 05:10

bmu

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answered Apr 24 '12 at 12:31

DSM

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Great. Exactly what I wanted! – user989762 Apr 24 '12 at 14:13
Do you know how to know the length of the cap in data units? https://stackoverflow.com/questions/69858359/what-is-the-length-of-the-cap – Nov 05 '21 at 19:40

Linewidth is added to the length of a line

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