The documentation for Facet Mark Options states:
Faceting can be explicitly enabled or disabled on a mark with the facet option, which accepts the following values:
[…]
- null (or false) - repeat this mark’s data across all facets (i.e., no faceting)
I took this to mean that Plot.axisX({facet: null}) repeats the x axis on each facet. However, it does not. The correct property to use is facetAnchor. I found that confusing.