[matplotlib-devel] bug in vector plotting

[David R. <ra...@ke...>]

I think that I have found a serious bug in vector plotting (quiver)
when the "angle=xy" option is used on a plot with a large aspect
ratio.  It is my understanding that with this option, the vector with
tail at (x,y) would have its head at (x + vx,y + vy); at least this is
what it says in the documentation.

The problem is best illustrated by the following example:

------------

#!/usr/bin/python
#
from numpy import *
import matplotlib.pyplot as plt

lx = 100.
ly = 10.
pi = 3.14159
kx = pi/lx
ky = pi/ly
nx = 21
ny = 21
x = linspace(0,lx,nx)
y = linspace(0,ly,ny)
(X,Y) = meshgrid(x,y)

# this is a streamfunction
psi = sin(kx*X)*sin(ky*Y)

# these are the velocity components derived from the streamfunction
vx = ky*sin(kx*X)*cos(ky*Y)
vy = -kx*cos(kx*X)*sin(ky*Y)

# plot the x velocity
cx = plt.contourf(x,y,vx)
b = plt.colorbar(cx, orientation='vertical')
tl = plt.title("vx")
xlab = plt.xlabel("x")
ylab = plt.ylabel("y")
plt.show()

# plot the y velocity
cy = plt.contourf(x,y,vy)
b = plt.colorbar(cy, orientation='vertical')
tl = plt.title("vy")
xlab = plt.xlabel("x")
ylab = plt.ylabel("y")
plt.show()

# plot the streamfunction and the velocity vectors using the angles=xy option
cp = plt.contour(x,y,psi)
q = plt.quiver(X,Y,vx,vy,angles='xy')
tl = plt.title("psi contours, (vx,vy) vectors")
xlab = plt.xlabel("x")
ylab = plt.ylabel("y")
plt.show()

------------------------------------

The contour plot of vy, the y component of the vector, clearly shows
that vy is non-zero at y = 5.  However, the vector plot has it zero
along this line.  Interestingly, vx appears to be represented correctly
on the x = 50 line.  The magnitude of vectors should be inversely
proportional to the spacing of streamfunction contours, and this is
manifestly untrue in the vector plot.

Please tell me if I am doing something stupid.

Dave

PS: I am using matplotlib 1.1.1 and numpy 1.6.2 on Arch linux.  (Yes,
I have to set the #!/bin/python line in the example to #!/bin/python2
on Arch, since Arch has python3 as default!)

-- 
David J. Raymond
Prof. of Physics
New Mexico Tech
http://www.physics.nmt.edu/~raymond/index.html