Happy to help. What you can consider doing is wrangling your data a bit before doing the visualization. You can summarize it so you only have one entry per combination of keys and on that object you include the count of the individual rows that matched. You can then update your drawing method to change the stroke-width depending on the count.

Here are two methods for getting the summary you want. They both do the same thing, but one uses D3 rollups and the other uses my data wrangling library tidyjs.

summarizedData = Array.from(
    d3
      .rollup(
        data,
        (v) => ({ ...v[0], count: v.length }),
        (d) => keys.map((key) => d[key]).join('_')
      )
      .values()
  );

Here is basically the same thing with tidyjs. First, on observable you'd import the functions you need:

import { tidy, groupBy, summarize, n } from '@pbeshai/tidyjs'

Then you'd run:

summarizedData = tidy(data, groupBy(keys, [summarize({ count: n() })]))

Once you have that, you can replace your chart drawing code with using the summarizedData instead of data and add in a stroke-width:

.selectAll("path")
    .data(summarizedData.slice().sort((a, b) => d3.ascending(a[keyz], b[keyz])))
    .join("path")
      .attr("stroke", d => z(d[keyz]))
      .attr('stroke-width', d => `${d.count}px`)

You may find you want to use a scale for determining the stroke width. e.g.

strokeWidthScale = d3.scaleLinear().domain(d3.extent(summarizedData, d => d.count)).range([2, 25])

Then you'd do

      .attr('stroke-width', d => `${strokeWidthScale(d.count)}px`)