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/round.go

@@ -0,0 +1,62 @@
+package chart
+
+import (
+	"fmt"
+	"math"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+// turn is a dot's angle, 0–1 clockwise from twelve.
+func turn(dx, dy int) float64 {
+	a := math.Atan2(float64(dx)+0.5, -float64(dy)-0.5) / (2 * math.Pi)
+	if a < 0 {
+		a++
+	}
+	return a
+}
+
+// sector fills dots between radii r0–r1 and turns a0–a1.
+func sector(g pixel.Canvas, cx, cy, r0, r1 int, a0, a1 float64, s pixel.Slot) {
+	for dy := -r1; dy <= r1; dy++ {
+		for dx := -r1; dx <= r1; dx++ {
+			d := dx*dx + dy*dy
+			if d > r1*r1+r1 || d < r0*r0 {
+				continue
+			}
+			if a := turn(dx, dy); a >= a0 && a < a1 {
+				g.Dot(cx+dx, cy+dy, s)
+			}
+		}
+	}
+}
+
+// shares are slices' fractions of their total, checked.
+func shares(slices []Slice) (total float64, cuts []float64) {
+	for _, s := range slices {
+		if s.Value < 0 || math.IsNaN(s.Value) || math.IsInf(s.Value, 0) {
+			panic(fmt.Sprintf("chart: slice %q has value %v", s.Name, s.Value))
+		}
+		total += s.Value
+	}
+	if total == 0 {
+		panic("chart: slices sum to zero")
+	}
+	at := 0.0
+	cuts = []float64{0}
+	for _, s := range slices {
+		at += s.Value / total
+		cuts = append(cuts, at)
+	}
+	cuts[len(cuts)-1] = 1
+	return total, cuts
+}
+
+// readSlices lists slices for assistive technology.
+func readSlices(slices []Slice) string {
+	labels, values := make([]string, len(slices)), make([]float64, len(slices))
+	for i, s := range slices {
+		labels[i], values[i] = s.Name, s.Value
+	}
+	return read(labels, values)
+}