def _format_time_axis(ax, gps, span):
    """Format the time axis of an omega scan plot

    Parameters
    ----------
    ax : `~matplotlib.axis.Axis`
        the `Axis` object to format

    gps : `float`
        reference GPS time (in seconds) to serve as the origin

    span : `float`
        total duration (in seconds) of the time axis
    """
    ax.set_xscale('seconds', epoch=gps)
    ax.set_xlim(gps - span / 2, gps + span / 2)
    ax.grid(True, axis='x', which='major')
