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/stackedbar.go

@@ -0,0 +1,50 @@
+package chart
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type StackedBarOptions struct {
+	// Width and Height are dots; zeros mean 160 by 80.
+	Width, Height int
+}
+
+// StackedBar stacks each category's series, one slot per series.
+func StackedBar(c *ui.Context, text string, labels []string, series []Series, o StackedBarOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(labels) == 0 {
+		panic("chart: StackedBar has no labels")
+	}
+	w, h := size("StackedBar", o.Width, o.Height, 160, 80)
+	totals := make([]float64, len(labels))
+	for _, s := range series {
+		if len(s.Values) != len(labels) {
+			panic(fmt.Sprintf("chart: series %q has %d values for %d labels", s.Name, len(s.Values), len(labels)))
+		}
+		for i, v := range s.Values {
+			if v < 0 {
+				panic(fmt.Sprintf("chart: series %q stacks a negative %v", s.Name, v))
+			}
+			totals[i] += v
+		}
+	}
+	_, hi := span(totals)
+	p := fit(0, hi, w, h, labels)
+	return ui.Box(c).Size(k.D(w), k.D(h)).Role(ui.RoleImage).Label(text).Description(readSeries(labels, series)).Draw(func(pt *ui.Painter, r ui.Rect) {
+		g := k.Canvas(pt, r)
+		p.grid(g, labels)
+		bw := max(1, p.w/len(labels)*2/3)
+		for i := range labels {
+			x, base := p.slot(i, len(labels))-bw/2, 0.0
+			for j, s := range series {
+				lo, top := p.at(base), p.at(base+s.Values[i])
+				g.Rect(x, top, bw, lo-top, pick(s.Slot, j))
+				base += s.Values[i]
+			}
+		}
+	})
+}