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/histogram.go

@@ -0,0 +1,54 @@
+package chart
+
+import (
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type HistogramOptions struct {
+	// Bins is roughly how many, zero for 8; edges round.
+	Bins int
+	// Width and Height are dots; zeros mean 160 by 80.
+	Width, Height int
+	Slot          pixel.Slot
+}
+
+// Histogram counts values into bins, side by side.
+func Histogram(c *ui.Context, text string, values []float64, o HistogramOptions) *ui.Element {
+	k := pixel.K(c)
+	n := pixel.Or("Histogram Bins", o.Bins, 8)
+	w, h := size("Histogram", o.Width, o.Height, 160, 80)
+	if len(values) == 0 {
+		panic("chart: Histogram has no values")
+	}
+	lo, hi := span(values)
+	// A lone value gets a bin sized to itself.
+	run := hi - lo
+	if run == 0 {
+		run = max(math.Abs(lo), 1)
+	}
+	step := nice(run / float64(n))
+	from := math.Floor(lo/step) * step
+	bins := int((hi-from)/step) + 1
+	counts, labels := make([]float64, bins), make([]string, bins)
+	for _, v := range values {
+		counts[int((v-from)/step)]++
+	}
+	for i := range bins {
+		labels[i] = tick(from+float64(i)*step, step)
+	}
+	_, top := span(counts)
+	p := fit(0, top, w, h, labels)
+	ink := pick(o.Slot, 0)
+	return ui.Box(c).Size(k.D(w), k.D(h)).Role(ui.RoleImage).Label(text).Description(read(labels, counts)).Draw(func(pt *ui.Painter, r ui.Rect) {
+		g := k.Canvas(pt, r)
+		p.grid(g, labels)
+		for i, v := range counts {
+			x0, x1 := p.x+i*p.w/bins, p.x+(i+1)*p.w/bins
+			g.Rect(x0, p.at(v), x1-x0-1, p.at(0)-p.at(v), ink)
+		}
+	})
+}