def update_graphs_header(pathname):
    """
    Update the graphs page header on page load or refresh.

    Parameters
    ----------
    pathname : str
        The pathname of the url in window.location

    Returns
    -------
    header : str
        The header for the graphs page that includes the hostname
    desc: str
        A description of what the graphs page shows
    """
    # Parse hostname.
    hostname = pathname.split('/')[1]
    header = hostname + ' Graphs'

    desc = ('This page features two histograms that will help you explore '
            'and visualize image data from the {} device.'.format(hostname))

    return header, desc