Re: [Matplotlib-users] scaling arrows in quiver

jas...@cr... wrote:
> A couple of us are trying to figure out how to scale arrows in a quiver 
> plot so that we can exactly specify what the output arrows look like.  
> For example, we'd like to scale the vectors to half of their size, and 
> have it look like that on the quiver plot.
> 
> So I tried even just getting a quiver plot to plot an arrow exactly as I 
> passed it, without it scaling anything.  My attempt is this:
> 
> import matplotlib.pylab as plt
> fig=plt.figure(figsize=[6,6])
> q=plt.quiver([0],[0],[1],[1],units='x',scale=1,angles='xy')
> ax=plt.axis([0,1.5,0,1.5])
> plt.grid(True)
> plt.savefig('test.png')
> 
> I'm trying to get the arrow to go from (0,0) to (1,1).  However, with 
> units='x', it's just short, and with units='y', it's just a bit too 
> long.  Furthermore, if I don't make the aspect ratio equal to one, I get 
> wild results since the x-axis and y-axis are different units then.  It 
> would be really nice if there was a way to say units='data' (for data 
> coordinates), and then if scale=1, the arrows would be drawn with the 
> heads and tails at exactly the passed points.  If scale=2, then each 
> arrow would be drawn exactly half of its length.  I realize that 
> units='data' might mess up the other measurements of an arrow, though, 
> so maybe another parameter is called for, like a scale_units, that 
> defaults to units.

This sounds like a good idea.  The present situation does the originally 
intended autoscaling of arrows reasonably well, but it is very confusing 
when you need more control.  A scale_units parameter could make it much 
easier to figure out how to manually set the scale.  I will look into 
it.  It might be trivial, it might not.

Eric