How does Python's matplotlib.pyplot.quiver exactly work?

Question

I'm trying to understand how the quiver function in the NumPy module works. Supposedly it allows to visualize graphically the values of two arrays, for example horizontal and vertical velocities. I have the following very simple example, but I show it just to see if you can help me to find out what I'm not doing well:

x = np.linspace(0,1,11)
y = np.linspace(1,0,11)
u = v = np.zeros((11,11))
u[5,5] = 0.2

plt.quiver(x, y, u, v)

The code produces the following figure:

As you can see, the arrow is not an arrow, but a line and it is longer than 0.2. My intention is to get an arrow of length 0.2 and I thought I could do it using quiver. Is it possible? Or should I better use another command?

Answer 1

matplotlib quiver does auto scaling. Set the scale to 1 to get your 0.2 units in x an y:

x = np.linspace(0,1,11)
y = np.linspace(1,0,11)
u = v = np.zeros((11,11))
u[5,5] = 0.2

plt.quiver(x, y, u, v, scale=1)

If you don't set scale, matplotlib uses an auto scaling algorithm based on the average vector length and the number of vectors. Since you only have one vector with a length greater zero, it becomes really big. Adding more vectors makes the arrows successively smaller.

To have equal x and y extensions of your arrow a few more adjustments are needed:

x = np.linspace(0,1,11)
y = np.linspace(1,0,11)
u = v = np.zeros((11,11))
u[5,5] = 0.2

plt.axis('equal')
plt.quiver(x, y, u, v, scale=1, units='xy')

Both axes need to be equal and the units need to be set to xy.