Add support for ellipse() ? about d3-path

[nitaku]

I would expect this signature for the ellipse method:

ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, counterclockwise)

as described here in more detail: https://html.spec.whatwg.org/multipage/canvas.html#dom-context-2d-ellipse
It would be identical to the ellipse method defined for canvas.

[mbostock]

Is ellipse new? Doesn’t look like part of CanvasPathMethods:

https://www.w3.org/TR/2dcontext/#canvaspathmethods

[nitaku]

I found it in this MDN documentation page, which lead to the HTML Living Standard spec.

I am not 100% sure about the status of these specifications, but the W3C one you mentioned is marked as "superseded". A level 2 spec was in the working, but it seems to be abandoned: https://www.w3.org/TR/2dcontext2/

I tested ellipse() with canvas on Chrome 88. It also seems to be widely adopted, according to https://caniuse.com/mdn-api_canvasrenderingcontext2d_ellipse

[nitaku]

I hope it could be a matter of adapting the code to accept two different radii, given that the A command in an SVG path definition already represents an elliptical arc (and providing an ellipse() method, which could be also leveraged by arc()).

[curran]

@nitaku Do you have an idea in mind for what the API could look like?

[nitaku]

We used a few test renderings made with Canvas as a reference.
Here is a CodePen that compares Canvas and SVG renderings made with the modified code: https://codepen.io/nitaku/pen/dyvyGaZ