Understanding Matplotlib's quiver plotting

Question

I'm trying to understand how plt.quiver() works. My issue is as follows:

I plot a simple vector (1,1) as such:

import numpy as np
import matplotlib.pyplot as plt

fig = plt.figure(2)
ax = fig.add_subplot(111)
ax.quiver(0,0, 1, 1, units = 'xy', scale = 1)
plt.xticks(range(-5,6))
plt.yticks(range(-5,6))
plt.grid()

I would expect the arrow to go from (0,0) to (1,1), but the result is slightly off from that:

Similarly, I try and plot an arrow for vector (0,3) and the resulting arrow seems to be for vector (0,3.5)...

My assumption is that this has something to do with the kwargs 'units', 'scale', 'angles', & 'scale_units'. I've read the docs on them but don't fully understand how they work. A sunday school explanation would be greatly appreciated!

How about, you try them, plot them together with the plots you already have and report remaining questions? — Chiel
– Chiel, Commented Apr 10, 2016 at 19:20
used: ax.quiver((0,0), (0,0), (1,0), (1,3), units = 'xy', scale = 1) and got same result (just combined). ie a chart with 1 arrow going from 0,0 to 0,3.5(ish) and another arrow going from 0,0 to aprox 0.9,1.2 — RSHAP
– RSHAP, Commented Apr 10, 2016 at 19:24
... How so? Why don't the go to expected coords, i.e. The ones I supplied? — RSHAP
– RSHAP, Commented Apr 10, 2016 at 19:47
See the answer below about the aspect ratio. Then your problem is solved. — Chiel
– Chiel, Commented Apr 11, 2016 at 11:06

Reblochon Masque · Accepted Answer · 2016-04-11 05:43:36Z

8

If you adjust the aspect ratio of the figure to 1, the vectors are displayed to proper scale:

import numpy as np
import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(111)
ax.quiver((0,0), (0,0), (1,0), (1,3), units = 'xy', scale = 1)
plt.axis('equal')
plt.xticks(range(-5,6))
plt.yticks(range(-5,6))
plt.grid()
plt.show()

edited Apr 11, 2016 at 5:43

answered Apr 11, 2016 at 5:01

Reblochon Masque

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kanayamalakar · Accepted Answer · 2016-04-11 10:53:41Z

5

You can try this code.

import matplotlib.pyplot as plt

fig = plt.figure(2)
ax = fig.add_subplot(111)
ax.quiver(0,0, 1, 1,angles='xy', scale_units='xy', scale = 1)
plt.xticks(range(-5,6))
plt.yticks(range(-5,6))
plt.grid()
plt.draw()
plt.show()

Just remember that the first two arguments of quiver are the x and y coordinates of the tail of the vector, the next two are the lengths of the vector along x and y direction respectively. angle='xy' makes the arrow point from tail of the vector to its tip.

You can find out more about matplotlib.quiver at http://matplotlib.org/1.3.1/api/pyplot_api.html#matplotlib.pyplot.quiver

answered Apr 11, 2016 at 10:53

kanayamalakar

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