Can anyone help me understand this: the code below produces a plot in which the x and y axis labels are reversed relative to the direction in which z varies on the plot, although the gradient arrows are correctly oriented. Is there something I've missed in understanding the sense of the x and y directions in mgrid, imshow, and gradient?
"""
x,y axes seem to be reversed
"""
import numpy as np
from matplotlib.pylab import *
dx = .25; dy = .25
xlen = 2.0; ylen = 2.0
x,y = np.mgrid[-xlen:xlen:dx, -ylen:ylen:dy] #also works with ogrid
z = x
hold(True)
imshow(z, origin='lower', extent=[-xlen,xlen,-ylen,ylen])
colorbar()
[u,v] = np.gradient(z)
u /= dx; v /= dy
QP = quiver(x,y,u,v,pivot='middle',color=[0.5,0.5,0.5])
quiverkey(QP, 0.85, 1.05, 1.0, '1 unit', labelpos='N')
xlabel('x')
ylabel('y')
title('slope')
show()