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/gauge.go

@@ -0,0 +1,45 @@
+package chart
+
+import (
+	"fmt"
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type GaugeOptions struct {
+	// Min and Max bound the dial; zeros mean 0–100.
+	Min, Max float64
+	// Size is the dial's width in dots; zero means 64.
+	Size int
+}
+
+// Gauge needles a half dial and prints the value.
+func Gauge(c *ui.Context, text string, value float64, o GaugeOptions) *ui.Element {
+	k := pixel.K(c)
+	span([]float64{o.Min, o.Max, value})
+	lo, hi := o.Min, o.Max
+	if lo == 0 && hi == 0 {
+		hi = 100
+	}
+	if !(lo < hi && value >= lo && value <= hi) {
+		panic(fmt.Sprintf("chart: Gauge value %v outside %v–%v", value, lo, hi))
+	}
+	n := pixel.Or("Gauge Size", o.Size, 64)
+	shown := num(value)
+	t := (value - lo) / (hi - lo)
+	return ui.Box(c).Size(k.D(n), k.D(n/2+10)).Role(ui.RoleMeter).Label(text).Description(shown).Draw(func(p *ui.Painter, r ui.Rect) {
+		g := k.Canvas(p, r)
+		c, rad := n/2, n/2-1
+		// The dial runs from nine o'clock, over twelve, to three.
+		sector(g, c, c, rad-5, rad, 0.75, 1, pixel.Good)
+		sector(g, c, c, rad-5, rad, 0, 0.125, pixel.Warn)
+		sector(g, c, c, rad-5, rad, 0.125, 0.25, pixel.Bad)
+		a := math.Pi * (1 - t)
+		g.Line(c, c, c+int(math.Round(float64(rad-3)*math.Cos(a))), c-int(math.Round(float64(rad-3)*math.Sin(a))), pixel.Ink)
+		g.Disc(c, c, 2, pixel.Edge)
+		tiny(g, c-tinyW(shown)/2+1, c+5, shown, pixel.Ink)
+	})
+}