Generate the path to the visualization for bundles.

        @param charm_id The ID of the bundle.
        @param channel Optional channel name.
        @return The url to the visualization.

def bundle_visualization_url(self, bundle_id, channel=None):
        '''Generate the path to the visualization for bundles.

        @param charm_id The ID of the bundle.
        @param channel Optional channel name.
        @return The url to the visualization.
        '''
        url = '{}/{}/diagram.svg'.format(self.url, _get_path(bundle_id))
        return _add_channel(url, channel)