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/areachart.go

@@ -0,0 +1,41 @@
+package chart
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type AreaChartOptions struct {
+	// Width and Height are dots; zeros mean 160 by 80.
+	Width, Height int
+	Slot          pixel.Slot
+}
+
+// AreaChart draws a line with the area below it dithered.
+func AreaChart(c *ui.Context, text string, labels []string, values []float64, o AreaChartOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("AreaChart", o.Width, o.Height, 160, 80)
+	if len(values) != len(labels) || len(values) < 2 {
+		panic(fmt.Sprintf("chart: AreaChart has %d labels for %d values", len(labels), len(values)))
+	}
+	s := []Series{{Name: text, Values: values, Slot: pick(o.Slot, 0)}}
+	lo, hi := span(values)
+	p := fit(min(0, lo), max(0, hi), w, h, labels)
+	return ui.Box(c).Size(k.D(w), k.D(h)).Role(ui.RoleImage).Label(text).Description(read(labels, values)).Draw(func(pt *ui.Painter, r ui.Rect) {
+		g := k.Canvas(pt, r)
+		p.grid(g, labels)
+		// The area spans from the line to zero, either way.
+		n, base := len(values), p.at(0)
+		for i := 1; i < n; i++ {
+			x0, x1 := p.slot(i-1, n), p.slot(i, n)
+			for x := x0; x < x1; x++ {
+				y := p.at(values[i-1] + (values[i]-values[i-1])*float64(x-x0)/float64(x1-x0))
+				g.Veil(x, min(y, base), 1, abs(y-base), s[0].Slot, 6)
+			}
+		}
+		lines(g, p, n, s)
+	})
+}