[Matplotlib-users] Plotting a circle while also changing the limits of the axes

[Brad M. <bra...@gm...>]

Hello,

I am trying to plot some small circles in my plotting window, in addition
to the curves I'm already plotting. If I don't want to set my x- and y-
axis scales equal to each other, a naive drawing of a circle results in an
ellipse.  To fix this problem I found some nice example code online here :
http://stackoverflow.com/questions/9230389/why-is-matplotlib-plotting-my-circles-as-ovals,
which solves the problem by basically plotting an ellipse, but an ellipse
which will look like a circle in the display window.

That works all fine for me, but then, if I change my xlim or ylim using
ax1.set_xlim((something1,something2)) then the solution no longer works,
and I get an ellipse.

A minimal example showing the breaking behavior can be seen below.



import matplotlib.pyplot as plt
> from matplotlib.patches import Ellipse, Circle
>
> fig = plt.figure()
> ax1 = fig.add_subplot(111)
> # uncomment the following line to see it break
> #ax1.set_xlim((0.2,1))
>
> # calculate asymmetry of x and y axes:
> x0, y0 = ax1.transAxes.transform((0, 0)) # lower left in pixels
> x1, y1 = ax1.transAxes.transform((1, 1)) # upper right in pixes
> dx = x1 - x0
> dy = y1 - y0
> maxd = max(dx, dy)
> width = .15 * maxd / dx
> height = .15 * maxd / dy
> # a circle you expect to be a circle, but it is not
> ax1.add_artist(Circle((.5, .5), .15))
> # an ellipse you expect to be an ellipse, but it's a circle
> ax1.add_artist(Ellipse((.75, .75), width, height))
>
> plt.show()


I suppose the problem is that ax1.transAxes.transform commands return the
same numbers, regardless of whether I've changed the limits or not. Is
there an easy and clean way to fix this (perhaps a different command for
getting x0,y0,x1, and y1)?

Thanks for the help!

Best,
Brad