 # meaning of scale in quiver

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
jslavin at cfa.harvard.edu 60 Garden Street, MS 83
phone: (617) 496-7981 Cambridge, MA 02138-1516
cell: (781) 363-0035 USA
________________________________________________________
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/matplotlib-users/attachments/20180301/f28c00ec/attachment.html>

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
jslavin at cfa.harvard.edu <mailto:jslavin at cfa.harvard.edu>??? ?? 60 Garden
Street, MS 83
phone: (617) 496-7981??? ?? Cambridge, MA 02138-1516
cell: (781) 363-0035 ? ? ? ? ? ? USA
________________________________________________________

_______________________________________________
Matplotlib-users mailing list
Matplotlib-users at python.org
https://mail.python.org/mailman/listinfo/matplotlib-users