matplotlib-users

[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

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

[Benjamin R. <ben...@ou...>]

On Mon, Nov 5, 2012 at 3:51 PM, Brad Malone <bra...@gm...> wrote:

> 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.
>

I am probably gonna reply to that stackoverflow question with a better
response...

Essentially, you want a similar behavior to the markers in the scatter
plots, right?  As you zoom or resize the plot, the circle markers stay as
circles and have the same size relative to the size of the figure.  If that
is what you want, the way to do that is very easy.

ax1.scatter([0.5], [0.5], s=30)

Or whatever size you want (units of points).

I hope that helps!
Ben Root

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

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

>
>
>
> Essentially, you want a similar behavior to the markers in the scatter
> plots, right?  As you zoom or resize the plot, the circle markers stay as
> circles and have the same size relative to the size of the figure.  If that
> is what you want, the way to do that is very easy.
>
> ax1.scatter([0.5], [0.5], s=30)
>
> Or whatever size you want (units of points).
>
> I hope that helps!
> Ben Root
>
>
Hi Ben,

Thanks. I actually tried this before and it didn't appear to work for me.
But I think the problem was that I thought s was the radius in units of my
axes, and so was simply not seeing the dot on top of the line that was
already there. I was just choosing values that were way too small to see.

This should meet my needs.

I wonder if you would mind expanding on that sentence? Does the example I
> provide do what you want?
>
> Thanks,
> Phil


Phil, thanks for this example as well. It also would work for my purposes.
All I meant by that sentence is that, in my real script, I am plotting a
bunch of lines as well, and I just wanted these circles to also be present
in the plot (the example code I attached was simply the circles/ellipses).


Best,
Brad

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

[Jason G. <jas...@cr...>]

On 11/5/12 3:19 PM, Benjamin Root wrote:
>
>
> On Mon, Nov 5, 2012 at 3:51 PM, Brad Malone <bra...@gm...
> <mailto:bra...@gm...>> wrote:
>
>     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.
>
>
> I am probably gonna reply to that stackoverflow question with a better
> response...
>
> Essentially, you want a similar behavior to the markers in the scatter
> plots, right?  As you zoom or resize the plot, the circle markers stay
> as circles and have the same size relative to the size of the figure.
> If that is what you want, the way to do that is very easy.
>
> ax1.scatter([0.5], [0.5], s=30)
>
> Or whatever size you want (units of points).

I think the units are points^2, i.e., area of the circle...

Thanks,

Jason

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

[Benjamin R. <ben...@ou...>]

On Mon, Nov 5, 2012 at 4:22 PM, Jason Grout <jas...@cr...>wrote:

> On 11/5/12 3:19 PM, Benjamin Root wrote:
> >
> >
> > On Mon, Nov 5, 2012 at 3:51 PM, Brad Malone <bra...@gm...
> > <mailto:bra...@gm...>> wrote:
> >
> >     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.
> >
> >
> > I am probably gonna reply to that stackoverflow question with a better
> > response...
> >
> > Essentially, you want a similar behavior to the markers in the scatter
> > plots, right?  As you zoom or resize the plot, the circle markers stay
> > as circles and have the same size relative to the size of the figure.
> > If that is what you want, the way to do that is very easy.
> >
> > ax1.scatter([0.5], [0.5], s=30)
> >
> > Or whatever size you want (units of points).
>
> I think the units are points^2, i.e., area of the circle...
>
> Thanks,
>
> Jason
>
>
Right, thank you.  I keep forgetting that.

Ben Root

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

[Phil E. <pel...@gm...>]

Hi Brad,

I didn't quite follow what it was that you were trying to achieve, but the
following example may be of interest to you:


import matplotlib.pyplot as plt

from matplotlib.patches import Ellipse, Circle

import matplotlib.transforms as mtrans


fig = plt.figure()

ax1 = fig.add_subplot(111)


x_in_axes_coords, y_in_axes_coords = 0.5, 0.5

radius_in_axes = 0.3

coords = [[x_in_axes_coords, y_in_axes_coords],

          [x_in_axes_coords + radius_in_axes, y_in_axes_coords]]


coords = ax1.transAxes.transform(coords)

x_device, y_device = coords[0, :]

radius = coords[1, 0] - x_device


circle = Circle((x_device, y_device), radius,

                  transform=mtrans.IdentityTransform())


fig.artists.append(circle)



plt.show()



Clearly, you will always have circles with this approach (the circle is
defined in device coordinates, i.e. pixels),
but with the way this is implemented, it does not behave in the same way as
axes coordinates do when you resize your window.


> I am trying to plot some small circles in my plotting window, in addition
to the curves I'm already plotting.

I wonder if you would mind expanding on that sentence? Does the example I
provide do what you want?


Thanks,

Phil



On 5 November 2012 20:51, Brad Malone <bra...@gm...> wrote:

> 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
>
>
>
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