[Matplotlib-users] meaning of scale in quiver

[Eric Firing]

Jon,

The relevant parameters are 'scale_units' (not 'units') and 'scale'. I 
think the docstring entry for 'scale_units' will clarify the situation:

scale_units : [ 'width' | 'height' | 'dots' | 'inches' | 'x' | 'y' | 
'xy' ], \
None, optional
     If the *scale* kwarg is *None*, the arrow length unit. Default is 
*None*.

     e.g. *scale_units* is 'inches', *scale* is 2.0, and
     ``(u,v) = (1,0)``, then the vector will be 0.5 inches long.

     If *scale_units* is 'width'/'height', then the vector will be half the
     width/height of the axes.

     If *scale_units* is 'x' then the vector will be 0.5 x-axis
     units. To plot vectors in the x-y plane, with u and v having
     the same units as x and y, use
     ``angles='xy', scale_units='xy', scale=1``.



In other words,

data_value * scale_units / scale = size in scale_units

I suspect it would have been less confusing if scale had been defined as 
the reciprocal of what it is--but we are stuck with it as-is.

Eric

On 2018/03/01 10:53 AM, Slavin, Jonathan wrote:
> Hi,
> 
> I'm making a plot in which I plot a bunch of velocity vectors that show 
> the deviation of the material from it's average velocity.  I use 
> quiverkey to show the size scale of the vectors, but I'd also like to 
> show the mean velocity vector on the plot.
> So, what I need is the meaning of the scale value. I'm using 
> units='dots' and have set the scale value by trial and error to 0.75 to 
> get a good looking plot.  Given these, can someone tell me how to 
> translate some value from a magnitude for a vector to a length on my 
> plot?  I've looked at the code for quiver and quiverkey and it's a bit 
> opaque to me, at least without spending significant time studying it.
> 
> Regards,
> Jon
> 
> -- 
> ________________________________________________________
> Jonathan D. Slavin                 Harvard-Smithsonian CfA
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