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/waterfall.go

@@ -0,0 +1,46 @@
+package chart
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type WaterfallOptions struct {
+	// Width and Height are dots; zeros mean 160 by 80.
+	Width, Height int
+}
+
+// Waterfall walks a running total: Good up, Bad down.
+func Waterfall(c *ui.Context, text string, labels []string, deltas []float64, o WaterfallOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(labels) != len(deltas) || len(deltas) == 0 {
+		panic(fmt.Sprintf("chart: Waterfall has %d labels for %d steps", len(labels), len(deltas)))
+	}
+	w, h := size("Waterfall", o.Width, o.Height, 160, 80)
+	totals := make([]float64, len(deltas)+1)
+	for i, d := range deltas {
+		totals[i+1] = totals[i] + d
+	}
+	lo, hi := span(totals)
+	p := fit(lo, hi, w, h, labels)
+	return ui.Box(c).Size(k.D(w), k.D(h)).Role(ui.RoleImage).Label(text).Description(read(labels, deltas)).Draw(func(pt *ui.Painter, r ui.Rect) {
+		g := k.Canvas(pt, r)
+		p.grid(g, labels)
+		bw := max(1, p.w/len(deltas)*2/3)
+		for i, d := range deltas {
+			a, b := p.at(totals[i]), p.at(totals[i+1])
+			ink := pixel.Good
+			if d < 0 {
+				ink = pixel.Bad
+			}
+			x := p.slot(i, len(deltas)) - bw/2
+			g.Rect(x, min(a, b), bw, max(1, abs(a-b)), ink)
+			if i+1 < len(deltas) {
+				g.Checker(x+bw, b, p.slot(i+1, len(deltas))-bw/2-x-bw, 1, pixel.Edge)
+			}
+		}
+	})
+}