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/candlestick.go

@@ -0,0 +1,64 @@
+package chart
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+// Candle is a period's open, high, low and close.
+type Candle struct {
+	Label                  string
+	Open, High, Low, Close float64
+}
+
+type CandlestickOptions struct {
+	// Width and Height are dots; zeros mean 160 by 80.
+	Width, Height int
+}
+
+// Candlestick draws wicks and bodies: up Good, down Bad.
+func Candlestick(c *ui.Context, text string, candles []Candle, o CandlestickOptions) *ui.Element {
+	k := pixel.K(c)
+	if len(candles) == 0 {
+		panic("chart: Candlestick has no candles")
+	}
+	w, h := size("Candlestick", o.Width, o.Height, 160, 80)
+	labels, lows, highs := make([]string, len(candles)), make([]float64, len(candles)), make([]float64, len(candles))
+	for i, cd := range candles {
+		span([]float64{cd.Open, cd.High, cd.Low, cd.Close})
+		if cd.Low > min(cd.Open, cd.Close) || cd.High < max(cd.Open, cd.Close) {
+			panic(fmt.Sprintf("chart: candle %q has open %v and close %v outside %v–%v", cd.Label, cd.Open, cd.Close, cd.Low, cd.High))
+		}
+		labels[i], lows[i], highs[i] = cd.Label, cd.Low, cd.High
+	}
+	lo, _ := span(lows)
+	_, hi := span(highs)
+	p := fit(lo, hi, w, h, labels)
+	return ui.Box(c).Size(k.D(w), k.D(h)).Role(ui.RoleImage).Label(text).Description(ohlc(candles)).Draw(func(pt *ui.Painter, r ui.Rect) {
+		g := k.Canvas(pt, r)
+		p.grid(g, labels)
+		bw := max(1, p.w/len(candles)/2)
+		for i, cd := range candles {
+			x, ink := p.slot(i, len(candles)), pixel.Good
+			if cd.Close < cd.Open {
+				ink = pixel.Bad
+			}
+			g.VLine(x, p.at(cd.High), p.at(cd.Low)-p.at(cd.High)+1, pixel.Edge)
+			top, bottom := p.at(max(cd.Open, cd.Close)), p.at(min(cd.Open, cd.Close))
+			g.Rect(x-bw/2, top, max(1, bw), max(1, bottom-top), ink)
+		}
+	})
+}
+
+// ohlc lists each candle's open, high, low and close.
+func ohlc(candles []Candle) string {
+	parts := make([]string, len(candles))
+	for i, cd := range candles {
+		parts[i] = cd.Label + " " + nums([]float64{cd.Open, cd.High, cd.Low, cd.Close})
+	}
+	return strings.Join(parts, "; ")
+}