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/linechart.go

@@ -0,0 +1,64 @@
+package chart
+
+import (
+	"fmt"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type LineChartOptions struct {
+	// Width and Height are dots; zeros mean 160 by 80.
+	Width, Height int
+}
+
+// LineChart draws series as Bresenham lines with square markers.
+func LineChart(c *ui.Context, text string, labels []string, series []Series, o LineChartOptions) *ui.Element {
+	k := pixel.K(c)
+	w, h := size("LineChart", o.Width, o.Height, 160, 80)
+	p := fitSeries(labels, series, w, h)
+	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)
+		lines(g, p, len(labels), series)
+	})
+}
+
+// fitSeries checks series against labels and fits their range.
+func fitSeries(labels []string, series []Series, w, h int) plot {
+	rows := make([][]float64, len(series))
+	for i, 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)))
+		}
+		rows[i] = s.Values
+	}
+	lo, hi := span(rows...)
+	return fit(lo, hi, w, h, labels)
+}
+
+// lines joins each series' points and marks them with squares.
+func lines(g pixel.Canvas, p plot, n int, series []Series) {
+	for j, s := range series {
+		ink := pick(s.Slot, j)
+		for i := 1; i < n; i++ {
+			g.Line(p.slot(i-1, n), p.at(s.Values[i-1]), p.slot(i, n), p.at(s.Values[i]), ink)
+		}
+		for i, v := range s.Values {
+			g.Rect(p.slot(i, n)-1, p.at(v)-1, 3, 3, ink)
+		}
+	}
+}
+
+// readSeries lists each series for assistive technology.
+func readSeries(labels []string, series []Series) string {
+	out := ""
+	for i, s := range series {
+		if i > 0 {
+			out += "; "
+		}
+		out += s.Name + ": " + read(labels, s.Values)
+	}
+	return out
+}