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/sparkline.go

@@ -0,0 +1,40 @@
+package chart
+
+import (
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type SparklineOptions struct {
+	// Width is in dots; zero means 48.
+	Width int
+	Slot  pixel.Slot
+}
+
+// Sparkline is a line one text line high, end marked.
+func Sparkline(c *ui.Context, text string, values []float64, o SparklineOptions) *ui.Element {
+	k := pixel.K(c)
+	w := pixel.Or("Sparkline Width", o.Width, 48)
+	if len(values) < 2 {
+		panic("chart: Sparkline needs two values or more")
+	}
+	lo, hi := span(values)
+	if hi == lo {
+		hi = lo + 1
+	}
+	ink := pick(o.Slot, 0)
+	at := func(i int) (int, int) {
+		return i * (w - 2) / (len(values) - 1), pixel.LineHeight - 3 - int((values[i]-lo)/(hi-lo)*float64(pixel.LineHeight-6))
+	}
+	return ui.Box(c).Size(k.D(w), k.D(pixel.LineHeight)).Role(ui.RoleImage).Label(text).Description(nums(values)).Draw(func(pt *ui.Painter, r ui.Rect) {
+		g := k.Canvas(pt, r)
+		for i := 1; i < len(values); i++ {
+			x0, y0 := at(i - 1)
+			x1, y1 := at(i)
+			g.Line(x0, y0, x1, y1, ink)
+		}
+		x, y := at(len(values) - 1)
+		g.Rect(x, y-1, 2, 2, pixel.Accent)
+	})
+}