Re: [Matplotlib-users] Not scaling patches.

[Ryan M. <rm...@gm...>]

On Thu, Aug 26, 2010 at 2:24 PM, Ben Edwards <bed...@cs...> wrote:
> Hello, I've used matplotlib for a while but never had cause to ask a
> question until now. I am trying to add a patch to an axis, but would like
> the patch to remain the same size when, interactively, I resize the
> resulting figure. I am using TkAgg, and trying the following:
>>>> import matplotlib.pyplot as plt
>>>> import matplotlib.patches as patches
>>>> ax = plt.gca()
>>>> cp = patches.Circle((.5,.5),.025)
>>>> ax.add_patch(cp)
>>>> ax.set_aspect('equal')
>>>> plt.draw()
>>>> plt.show()
> This gives me a nice blue circle in the middle of the plot, and using
> ax.set_aspect('equal') I know that it will not be distorted into an ellipse.
> But is there a way to ensure that when I make the Tk window bigger, that the
> circle appears to be the same size as it was previously? I assume this would
> entail some sort of transform, but I don't quite understand how they work...

You can make the circle this way, specifying everything in axes coordinates:

cp = patches.Circle((.5,.5),.025, transform=ax.transAxes)

However, this puts the circle in the same spot in axes coordinates
[(0.5,0.5) is the middle of the plot]. I'm not sure if that's what you
want.  I can't see any way to get Circle to use different transforms
for the center and the size, so from here the only path forward I see
is subclassing Circle. This way you could add code to the draw method
to calculate the center in axes coordinates from the center in data
coords. This needs to be done at draw time since the mapping of data
coords->axes coords changes as you pan and zoom.

Ryan

-- 
Ryan May
Graduate Research Assistant
School of Meteorology
University of Oklahoma