On Wed, Oct 28, 2009 at 9:55 AM, Tinne De Laet
<tinne.del...@mech.kuleuven.be> wrote:
> On Wed, Oct 28, 2009 at 9:06 AM, Eero Nevalainen
> <eero.nevalai...@indagon.com> wrote:
>> Hi,
>>
>> I need to draw error ellipses on a scatterplot. I'm guessing someone has
>> done this before.
>>
>> I've found some examples, such as this one
>> http://matplotlib.sourceforge.net/examples/pylab_examples/ellipse_rotated.html
>>
>> That led to the artist tutorial, and... ARGH! INFORMATION OVERFLOW!
>>
>> Can someone explain to me, why I suddenly have to know so much about
>> matplotlib's internals to get an ellipse drawn?
>
> Hi,
>
> I just made a function to draw uncertainty ellipses defined by a
> covariance matrix P:
>
> def plotEllipse(pos,P,edge,face):
> U, s , Vh = svd(P)
> orient = math.atan2(U[1,0],U[0,0])
> ellipsePlot = Ellipse(xy=pos, width=math.sqrt(s[0]),
> height=math.sqrt(s[1]), angle=orient,facecolor=face, edgecolor=edge)
> ax = gca()
> ax.add_patch(ellipsePlot);
> show()
> return ellipsePlot
>
> To use it: ellipsePlot=plotEllipse([x,y],P,'black','0.3')
>
> Hope this helps,
I still discoverd some problems with my plotEllipse function:
1) the angle in the ellipsePlot expects and angle in DEGREES and not
in radians apparently
2) forgot a factor 2 for the width and height (it's the entire width
not the `radius`)
3) removed the show() command which sometimes behaves strange (having
to close the figure before continuing plotting)
So a new trial:
def plotEllipse(pos,P,edge,face):
U, s , Vh = svd(P)
orient = math.atan2(U[1,0],U[0,0])*180/pi
ellipsePlot = Ellipse(xy=pos, width=2.0*math.sqrt(s[0]),
height=2.0*math.sqrt(s[1]), angle=orient,facecolor=face,
edgecolor=edge)
ax = gca()
ax.add_patch(ellipsePlot);
return ellipsePlot;
Good luck,
Tinne
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