docs: def callback(self, event):
          """
          Callback function to spawn a mini-browser when a feature is clicked.
          """
          artist = event.artist
          ind = artist.ind
          limit = 5
          browser = True
          if len(event.ind) > limit:
              print "more than %s genes selected; not spawning browsers" % limit
              browser = False
          for i in event.ind:
              feature = artist.features[ind[i]]
              print feature,
              if browser:
                  self.minibrowser.plot(feature)