Add chart components 216-245; step-aware ticks, font-aware labels

Bars, lines, areas, pies, donuts, radar, histogram, scatter, bubbles,
heatmaps, gauge, bullet, funnel, waterfall, treemap, candles, box
plots, steps, rings, legend, axis, crosshair, polar area, pyramid,
spectrum bars and win-loss strips, each drawn dot by dot. Ticks follow
1-2-5 steps and print as precisely as their step needs; a label row
uses micro letters or the pixel font as a whole. Descriptions carry the
data in numbers. Bad data panics.

chart/bulletchart.go

@@ -0,0 +1,45 @@
+package chart
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type BulletChartOptions struct {
+	// Width is in dots; zero means 160.
+	Width int
+}
+
+// BulletChart lays a bar on rising ranges and a target.
+func BulletChart(c *ui.Context, text string, value, target float64, ranges []float64, o BulletChartOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(ranges) == 0 {
+		panic("chart: BulletChart needs ranges")
+	}
+	span(ranges, []float64{value, target})
+	for i, r := range ranges {
+		if r <= 0 || i > 0 && r <= ranges[i-1] {
+			panic(fmt.Sprintf("chart: BulletChart ranges %v must rise above zero", ranges))
+		}
+	}
+	end := ranges[len(ranges)-1]
+	if value < 0 || value > end || target < 0 || target > end {
+		panic(fmt.Sprintf("chart: BulletChart value %v or target %v outside 0–%v", value, target, end))
+	}
+	w := pixel.Or("BulletChart Width", o.Width, 160)
+	x := func(v float64) int { return int(v / end * float64(w-1)) }
+	return ui.Box(c).Size(k.D(w), k.D(14)).Role(ui.RoleMeter).Label(text).Description(num(value) + " / " + num(target)).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		from := 0
+		for i, band := range ranges {
+			to := x(band) + 1
+			g.Dither(from, 0, to-from, 14, pixel.Well, pixel.Shade, 12-12*i/len(ranges))
+			from = to
+		}
+		g.Rect(0, 5, max(1, x(value)), 4, pixel.Ink)
+		g.Rect(min(x(target), w-2), 2, 2, 10, pixel.Accent)
+	})
+}