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/polararea.go

@@ -0,0 +1,36 @@
+package chart
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type PolarAreaOptions struct {
+	// Size is the chart's width in dots; zero means 64.
+	Size int
+}
+
+// PolarArea gives slices equal turns, reaching out by value.
+func PolarArea(c *ui.Context, text string, slices []Slice, o PolarAreaOptions) *ui.Element {
+	k := pixel.K(c)
+	n := pixel.Or("PolarArea Size", o.Size, 64)
+	shares(slices)
+	most := 0.0
+	for _, s := range slices {
+		most = max(most, s.Value)
+	}
+	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
+		g.Circle(c, c, rad, pixel.Shade)
+		g.Circle(c, c, rad/2, pixel.Shade)
+		step := 1 / float64(len(slices))
+		for i, s := range slices {
+			if s.Value > 0 {
+				reach := max(1, int(s.Value/most*float64(rad)))
+				sector(g, c, c, 0, reach, float64(i)*step, float64(i+1)*step, pick(s.Slot, i))
+			}
+		}
+	})
+}