provide some way to pre-filter splits & ticks

[leeoniya]

context: #304 (comment)

axis.values currently serves 2 purposes: tick formatting and tick filtering (in the case of log axes). it's possible to want to customize just the former without having to re-implement the latter. would be useful to have some intermediate step between the splits gen and the values gen. same maybe for gridlines.

something like:

{
axes: [
  {
    filter: (self, splits, ...) => valuesSplits,
    grid: {
      filter: (self, splits, ...) => linesSplits,
    }
}

another option would be to use the same strategy as scale ranging and simply expose the filters as static utility functions which can be used inside of custom axis.values:

e.g. uPlot.filtLog10():

uPlot/src/opts.js

Lines 435 to 457 in e2285cc

	const RE_ALL = /./;
	const RE_12357 = /[12357]/;
	const RE_125 = /[125]/;
	const RE_1 = /1/;
	export function logAxisVals(self, splits, axisIdx, foundSpace, foundIncr) {
	let axis = self.axes[axisIdx];
	let scaleKey = axis.scale;
	let valToPos = self.valToPos;
	let minSpace = axis.space(); // TOFIX: only works for static space:
	let _10 = valToPos(10, scaleKey);
	let re = (
	valToPos(9, scaleKey) - _10 >= minSpace ? RE_ALL :
	valToPos(7, scaleKey) - _10 >= minSpace ? RE_12357 :
	valToPos(5, scaleKey) - _10 >= minSpace ? RE_125 :
	RE_1
	);
	return splits.map(v => re.test(v) ? fmtNum(v) : "");
	}