Line thickness in VegaLite for mark: rule

Question

I have this plot and I am trying to make the dashed line really thin compared to the others, but can't quite figure it out. I've tried size on the actual mark-rule encoding part, as well as strokeWidth in the config (which at least the vega example here suggests is what i want) but no joy. What am I missing?

Answer 1

It appears that non-integer stroke widths are rounded up in rule marks; however this is not the case for line marks. If you replace your rule layer with this, it seems to do what you wish:

    {
      "data": {
        "values": [
          {"date": "2019-12-10", "metric": 100},
          {"date": "2019-12-16", "metric": 100}
        ]
      },
      "mark": {"type": "line", "strokeWidth": 0.5, "color": "black"},
      "encoding": {
        "x": {"field": "date", "type": "temporal", "timeUnit": "monthdate"},
        "y": {"field": "metric", "type": "quantitative"}
      }
    }

Here is the result (vega editor):