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/calendarheatmap.go

@@ -0,0 +1,62 @@
+package chart
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type CalendarHeatmapOptions struct {
+	// Slot is the busiest days' color; zero means Good.
+	Slot pixel.Slot
+}
+
+// day is a 4×4 square plus its gap, in dots.
+const day = 5
+
+// CalendarHeatmap rates a year's UTC days 0–4, week by week.
+func CalendarHeatmap(c *ui.Context, text string, year int, level func(d time.Time) int, o CalendarHeatmapOptions) *ui.Element {
+	k := pixel.K(c)
+	ink := o.Slot
+	if ink == 0 {
+		ink = pixel.Good
+	}
+	first := time.Date(year, 1, 1, 0, 0, 0, 0, time.UTC)
+	skip := int(first.Weekday())
+	levels := make([]int, first.AddDate(1, 0, 0).Sub(first)/(24*time.Hour))
+	sums := make([]float64, 12)
+	for i := range levels {
+		d := first.AddDate(0, 0, i)
+		l := level(d)
+		if l < 0 || l > 4 {
+			panic(fmt.Sprintf("chart: CalendarHeatmap level %d outside 0–4", l))
+		}
+		levels[i], sums[d.Month()-1] = l, sums[d.Month()-1]+float64(l)
+	}
+	months := strings.Fields(k.Msg("months"))
+	sm := small(months...)
+	head := labelH(months)
+	weeks := (skip + len(levels) + 6) / 7
+	// Assistive technology hears each month's summed levels.
+	return ui.Box(c).Size(k.D(weeks*day), k.D(head+7*day)).Role(ui.RoleImage).Label(text + " " + strconv.Itoa(year)).Description(read(months, sums)).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		// Month names top their first week, room permitting.
+		next := 0
+		for m, name := range months {
+			x := (skip + time.Date(year, time.Month(m+1), 1, 0, 0, 0, 0, time.UTC).YearDay() - 1) / 7 * day
+			if x >= next {
+				mark(g, x, 1, name, sm, pixel.Faint)
+				next = x + markW(name, sm) + 3
+			}
+		}
+		for i, l := range levels {
+			n := skip + i
+			g.Dither(n/7*day, head+n%7*day, day-1, day-1, pixel.Shade, ink, l*4)
+		}
+	})
+}