def graph_filename_url(site_id, base_graph_name):
    """This function returns a two-tuple: graph file name, graph URL.
    The graph file name is used to save the graph to the file system; the
    graph URL is used in an HTML site report to load the graph into an
    image tag.
    Parameters:
    'site_id': the Site ID of the site this graph is related to.
    'base_graph_name': a graph file name, not including the Site ID and not
        including the 'png' extension.  For example: 'eco_g1', which will
        produce a graph file name of 'ANSBG1_eco_g1.png' assuming
        the Site ID is ANSBG1.
    """
    fn = 'output/images/{}_{}.png'.format(site_id, base_graph_name)
    url = 'images/{}_{}.png'.format(site_id, base_graph_name)
    return fn, url