How to add a tick at a specific date to highlight it

I’m sorry to have to ask, I’m really new at this. I’ve made a little graph using ticks along a X axis of dates. The graph is split into 3 sections. I’d like to highlight Oct 3rd 2021 with a vertical line or tick in a different color/stroke, but I don’t know how to.

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Relevant code:

x: {

marks: [
          x: "Purchased At Date", 
          y: "Commerce Name", 
          stroke: "Commerce Name",
          strokeWidth: 1
marginLeft: 150,
marginBottom: 150

You could add a rule to your list of marks?

Plot.ruleX([new Date("2021-10-03")])

Or if you want a specific color:

Plot.ruleX([new Date("2021-10-03")], {stroke: "red"})
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