def fallbackSeries(requestContext, seriesList, fallback):
    """
    Takes a wildcard seriesList, and a second fallback metric.
    If the wildcard does not match any series, draws the fallback metric.

    Example:

    .. code-block:: none

      &target=fallbackSeries(server*.requests_per_second, constantLine(0))

    Draws a 0 line when server metric does not exist.

    """
    if len(seriesList) > 0:
        return seriesList
    else:
        return fallback