user-defined series path builder

While I’ve seen the note in README I wonder if you would consider at least accepting a user-supplied bezier-drawing function to smooth curves which is particularily nice to have on plots with only a few data points.

I’m no expert on canvas drawing, but I imagine for every line to render, you could call the user’s function using (xStart, yStart, xEnd, yEnd) arguments, do ctx.moveTo(xStart, yStart); and accept back the arguments to use on ctx.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y) to draw the curve.

[leeoniya]

updates:

series.draw is gone, replaced by series.paths: () => null. (candlestick demo updated, cc @ldstein)

the API for series.paths matches the internal buildPaths() [1] signature:

paths: (u, sidx, xdata, ydata, scaleX, scaleY) => {
  return {stroke, fill, clip};     // Path2D instances
}

a minor inconvenience remains with the fact that the internal buildPaths() is able to use getXPos() & getYPos() directly to get canvas offsets, but the exposed .valToPos() returns css pixel offsets, making you re-multiply by devicePixelRatio. you can see this already being done here:

https://github.com/leeoniya/uPlot/blob/01be7829ce6a743fb19dad2a930b8f66b57ff48e/demos/candlestick-ohlc.html#L143-L147

i should add extra methods, or a bool param to u.valToPos(val,"x", true) to directly return canvas pixels.

i also realized that this change is insufficient to fully convert the candlestick rendering in the demo because the the candles are effectively custom path “points”, for which i’ll soon add a similar-ish api as part of #10.

[1] https://github.com/leeoniya/uPlot/blob/master/src/Line.js#L682

[leeoniya]

uPlot now includes a smooth spline pathBuilder in the core, with a much better implementation than what’s in this demo.

https://leeoniya.github.io/uPlot/demos/line-paths.html