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/donutchart.go

@@ -0,0 +1,30 @@
+package chart
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type DonutChartOptions struct {
+	// Size is the ring's width in dots; zero means 64.
+	Size int
+}
+
+// DonutChart is a pie with a hole holding the total.
+func DonutChart(c *ui.Context, text string, slices []Slice, o DonutChartOptions) *ui.Element {
+	k := pixel.K(c)
+	n := pixel.Or("DonutChart Size", o.Size, 64)
+	total, cuts := shares(slices)
+	sum := num(total)
+	return ui.Box(c).Size(k.D(n), k.D(n)).Role(ui.RoleImage).Label(text).Description(readSlices(slices)).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		c, rad := n/2, n/2-1
+		for i, s := range slices {
+			sector(g, c, c, rad/2, rad, cuts[i], cuts[i+1], pick(s.Slot, i))
+		}
+		g.Circle(c, c, rad, pixel.Edge)
+		g.Circle(c, c, rad/2, pixel.Edge)
+		tiny(g, c-tinyW(sum)/2+1, c-2, sum, pixel.Ink)
+	})
+}