Modifying SVG path opacity and it's marker

Modifying the opacity instead of the stroke-opacity works.. so

d3.selectAll("path.link")
  .transition()
  .style("stroke-opacity", 0.5);

becomes

d3.selectAll("path.link")
  .transition()
  .style("opacity", 0.5);

You should be able to do the same for the marker path definition:

d3.selectAll("marker path")
  .transition()
  .style("stroke-opacity", 0.5);

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