def fieldmap_minmax(fieldmap, variable):
    """Data limits for a given varible across all zones in a fieldmap"""
    limits = None
    for z in fieldmap.zones:
        if limits is None:
            limits = z.values(variable).minmax()
        else:
            low, high = z.values(variable).minmax()
            limits = (min(limits[0], low), max(limits[1], high))
    return limits