cleaner area line option (WIP)

[Fil]

This still needs updates to the tests (to actually test that the line and fill options are going to the right place).

for #2133.

[mbostock]

This looks like a step in the right direction. I think we’ll probably want to lump the display attributes into three categories: (1) shared attributes that apply to both the area and line path, (2) attributes that only apply to the area path, and (3) attributes that only apply to the line path.

This branch currently produces something like:

<g>
  <g>
    <path aria-label="area" fill="#3ca951" d="M40,189.819L52.6,192.692…Z">
      <title>8 - Track Tape</title>
    </path>
    <path aria-label="area" fill="none" stroke="white" stroke-linejoin="round" stroke-linecap="round" transform="translate(0.5,0.5)" d="M40,189.819L52.6,192.692…">
      <title>8 - Track Tape</title>
    </path>
  </g>
</g>

But I think we are going to want something like:

<g aria-label="area">
  <g>
    <path fill="#3ca951" d="M40,189.819L52.6,192.692…Z"></path>
    <path fill="none" stroke="white" stroke-linejoin="round" stroke-linecap="round" transform="translate(0.5,0.5)" d="M40,189.819L52.6,192.692…"></path>
    <title>8 - Track Tape</title>
  </g>
</g>

In other words:

• Constant attributes such as aria-label should be moved up to the mark’s root G element.
• Shared attributes attributes such as mix-blend-mode and opacity should be on the G element that contains the per-instance area and line paths; this G element should also contain a singular TITLE element; the clip would also be applied here.
• Attributes that are specific to the area and line paths go on the path elements.

[mbostock]

Superseded by #2407.