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/funnel.go

@@ -0,0 +1,51 @@
+package chart
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type FunnelOptions struct {
+	// Width is in dots; zero means 160.
+	Width int
+}
+
+// Funnel narrows stage by stage, each bar with its count.
+func Funnel(c *ui.Context, text string, stages []Slice, o FunnelOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(stages) == 0 {
+		panic("chart: Funnel has no stages")
+	}
+	w := pixel.Or("Funnel Width", o.Width, 160)
+	top := stages[0].Value
+	for i, s := range stages {
+		span([]float64{s.Value})
+		if s.Value < 0 || i > 0 && s.Value > stages[i-1].Value {
+			panic(fmt.Sprintf("chart: Funnel stage %q has %v; stages must narrow from %v", s.Name, s.Value, top))
+		}
+	}
+	gutter := 0
+	for _, s := range stages {
+		gutter = max(gutter, pixel.Measure(s.Name)+6)
+	}
+	return ui.Box(c).Size(k.D(w), k.D(len(stages)*pixel.LineHeight)).Role(ui.RoleImage).Label(text).Description(readSlices(stages)).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		room := w - gutter
+		for i, s := range stages {
+			bw := max(2, int(part(s.Value, top)*float64(room)))
+			x, y := gutter+(room-bw)/2, i*pixel.LineHeight
+			g.Text(0, y, s.Name, pixel.Faint, 1)
+			g.Rect(x, y+2, bw, pixel.LineHeight-4, pick(s.Slot, i))
+			// The count sits on its bar, or beside thin ones.
+			n := num(s.Value)
+			if tinyW(n)+4 <= bw {
+				tiny(g, x+(bw-tinyW(n))/2, y+6, n, pixel.AccentInk)
+			} else {
+				tiny(g, x+bw+3, y+6, n, pixel.Ink)
+			}
+		}
+	})
+}