adding a vertical line to plot using seaborn object interface

Question

I am trying to use seaborn object interface. It looks intuitive like ggplot2 in R. However, since it is still in development stage, the underlying documentation is still WIP.

For example I am trying to add a vertical line at let's say x=5. How could I do so using the object interface.

   import seaborn.objects as so
    r=fmri[(fmri['event']=='stim') ].reset_index()
    (
        so.Plot(r, x="timepoint", y="signal",color='subject')
        .facet(row="region", wrap=2)
        .add(so.Line())
    )