title: Drawing ellipses on matplotlib basemap projections
body: <p>I am trying to draw ellipses on a basemap projection. To draw a circle like polygon there is the <code>tissot</code> function used to draw <a href="http://en.wikipedia.org/wiki/Tissot%27s_indicatrix">Tissot's indicatrix'</a> as illustrates the following example.</p>
  
  <pre><code>from mpl_toolkits.basemap import Basemap
  
  x0, y0 = 35, -50
  R = 5
  
  m = Basemap(width=8000000,height=7000000, resolution='l',projection='aea', 
      lat_1=-40.,lat_2=-60,lon_0=35,lat_0=-50)
  m.drawcoastlines()
  m.tissot(x0, y0, R, 100, facecolor='g', alpha=0.5)
  </code></pre>
  
  <p>However, I am interested in plotting an ellipsis in the form <code>(x-x0)**2/a**2 + (y-y0)**2/2 = 1</code>. On the other hand, to draw an ellipsis on a regular Cartesian grid I can use the following sample code:</p>
  
  <pre><code>import pylab
  from matplotlib.patches import Ellipse
  
  fig = pylab.figure()
  ax = pylab.subplot(1, 1, 1, aspect='equal')
  
  x0, y0 = 35, -50
  w, h = 10, 5
  e = Ellipse(xy=(x0, y0), width=w, height=h, linewidth=2.0, color='g')
  ax.add_artist(e)
  e.set_clip_box(ax.bbox)
  e.set_alpha(0.7)
  pylab.xlim([20, 50])
  pylab.ylim([-65, -35])
  </code></pre>
  
  <p>Is there a way to plot an ellipsis on a basemap projection with the an effect similar to <code>tissot</code>?</p>