Step curve could allow NaN x or y for discontinuities.

[mbostock]

Step curve could allow NaN x or y for discontinuities.

[mbostock]

This might require using a different trick to make the step curve symmetric:

var t = x > this._x ? this._t : 1 - this._t;

Perhaps something like curve.reverse() which returns a reverse curve to be used as the area’s baseline. If undefined, the curve is used.

[mbostock]

I’ve simplified the step curve in e3b1b0a, but there’s another issue that makes this difficult to implement: if the curve processes partially-defined points, it must use context.moveTo to render a discontinuity for a line, and likewise it must generate discontiguous area segments for an area.

It’s probably not possible for the step curve to produce discontiguous area segments internally; that’s primarily the responsibility of the area shape, which buffers the input coordinates so that it can pass the reversed xy coordinates of the baseline to the curve.

[mbostock]

I think you could maybe implement this by first detecting whether x or y was NaN inside the area and line shapes, and treating that as similar to the defined accessor returning falsey. Then, the shape would call curve.lineEnd (or maybe curve.point again?), passing in the partially-defined x and y, before terminating the line segment. That would give the curve the opportunity to draw the terminating tail to a partially-defined value, while keeping the responsibility to generate discontiguous line and area segments with the shape. You’d need to do something similar on curve.lineStart, passing in the last (possibly) partially-defined x and y before the first point in the new segment; this way the stepBefore curve could render a preceding horizontal line before the first defined value.