Add support for sankey links with arrows

[Andy2003]

This pull request adds a configuration to add arrows to the links in the sankey diagram.

Given the following configuration:

{
    "data": [
        {
            "type": "sankey",
            "node": {
                "pad": 5,
                "label": ["0", "1", "2", "3", "4", "5", "6"]
            },
            "link": {
                "arrowwidth": 20,
                "source": [
                    0, 0, 1, 2, 5, 4, 3
                ],
                "target": [
                    5, 3, 4, 3, 0, 2, 2
                ],
                "value": [
                    1, 2, 1, 1, 1, 1, 1
                ]
            }
        }],
    "layout": {
        "title": {"text": "Sankey with circular data and arrows"},
        "width": 800,
        "height": 800
    }
}

The following sankey diagram is generated:

[alexcjohnson]

Thanks @Andy2003 - that's a nice effect 😎

I'd call it a length though, since it's along the direction of the link, rather than a width which is typically perpendicular to the direction of the object. So arrowlen (we prefer not to use length in attribute names because of the built-in .length)

Two cases I'm curious about:

• Does this work for vertical Sankey diagrams (orientation: 'v')?
• What happens to non-circular links if the space between two nodes is close to or even less than the arrow length? Maybe we need to limit each arrow to take no more than perhaps half the available space?

[Andy2003]

Does this work for vertical Sankey diagrams (orientation: 'v')?

Yes:

What happens to non-circular links if the space between two nodes is close to or even less than the arrow length? Maybe we need to limit each arrow to take no more than perhaps half the available space?

Done:

[alexcjohnson]

Looks great! We'll need to figure out why the tests are failing, and let's see if @archmoj has any further comments.

[archmoj]

Please run the command below to commit the changes to plot-schema test.

npm run schema && git add test/plot-schema.json && git commit -m "update plot-schema diff"

There is a bug in our current image tests system.
To avoid that please rename those 3 new mocks and baselines so that they start with z-sankey_ and run in the end of the list.

Thanks.

[archmoj]

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LGTM.
We will merge this when we started adding features of 2.14.0.