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'....

Answer 1

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")