Searches a GenericElementPlot for a Link.

def find_link(cls, plot, link=None):
        """
        Searches a GenericElementPlot for a Link.
        """
        registry = Link.registry.items()
        for source in plot.link_sources:
            if link is None:
                links = [
                    l for src, links in registry for l in links
                    if src is source or (src._plot_id is not None and
                                         src._plot_id == source._plot_id)]
                if links:
                    return (plot, links)
            else:
                if ((link.target is source) or
                    (link.target is not None and
                     link.target._plot_id is not None and
                     link.target._plot_id == source._plot_id)):
                    return (plot, [link])