[Matplotlib-users] Possible bug (or unclear docs) with quiver

[Sergio <ser...@gm...>]

Hi,

trying to create a velocity plot for a 2d ode and superimposing it onto
a trajectory, I am encountering some issues with quiver.

Let ns1(x,t) be a vector function defining the ode, ready to be integrated with
scipy odeint and actually invariant with t. Here, x is a 1d state array with 2
entries.

I get the velocity field by first creating vectors of interesting points on the
two axis, say xg and yg. Then I make a grid with

xxg, yyg = np.meshgrid(xg, yg)

and I finally compute the velocity components as

vel = np.asarray(map(lambda x1, x2: ns1((x1,x2),0), xxg.flat, yyg.flat))
u=vel[:,0]
v=vel[:,1]

Now, I try to superimpose a trajectory with the velocity plot.
However, after having plotted the integrated trajectory, I cannot call

plt.quiver(xxg, yyg, u, v)

because the u and v components have very different ranges.

Trying to normalize as

plt.quiver(xxg, yyg, u/xrange, v/yrange)

looks (almost) right, but it is not nice to have to precompute the ranges.

I was expecting

plt.quiver(xxg, yyg, u, v, angles='xy')

to do the job automatically, but this is not the case.
For points with null x coordinate my arrows always have null length,
while this was not the case with the previous test.

any clue?