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/field.go

@@ -0,0 +1,69 @@
+package chart
+
+import (
+	"fmt"
+	"strings"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+// Point sits on two value axes; Size sizes bubbles.
+type Point struct {
+	X, Y, Size float64
+}
+
+// field is a plot with a value axis across too.
+type field struct {
+	plot
+	xfrom, xstep float64
+	xn           int
+}
+
+// fitPoints fits both axes around points, marks pad dots in.
+func fitPoints(points []Point, w, h, pad int) field {
+	if len(points) == 0 {
+		panic("chart: no points to plot")
+	}
+	xs, ys := make([]float64, len(points)), make([]float64, len(points))
+	for i, pt := range points {
+		xs[i], ys[i] = pt.X, pt.Y
+	}
+	xlo, xhi := span(xs)
+	ylo, yhi := span(ys)
+	f := field{plot: fit(ylo, yhi, w, h, []string{"0"})}
+	f.pad = pad
+	if f.w-1-2*pad < 2 || f.h-1-2*pad < 2 {
+		panic(fmt.Sprintf("chart: a %d×%d chart leaves no room inside %d-dot margins", w, h, pad))
+	}
+	f.xfrom, f.xstep, f.xn = ticks(xlo, xhi, 5)
+	return f
+}
+
+// ax is the x of v.
+func (f field) ax(v float64) int {
+	top := f.xfrom + float64(f.xn-1)*f.xstep
+	return f.x + f.pad + int((v-f.xfrom)/(top-f.xfrom)*float64(f.w-1-2*f.pad)+0.5)
+}
+
+// grid draws both axes with their ticks.
+func (f field) grid(g pixel.Canvas) {
+	f.plot.grid(g, nil)
+	for i := range f.xn {
+		v := f.xfrom + float64(i)*f.xstep
+		x, s := f.ax(v), tick(v, f.xstep)
+		g.Dot(x, f.y+f.h+1, pixel.Edge)
+		tiny(g, max(0, min(g.W-tinyW(s), x-tinyW(s)/2)), f.y+f.h+3, s, pixel.Faint)
+	}
+}
+
+// where lists points as "(x, y)", sizes after for bubbles.
+func where(points []Point, sized bool) string {
+	parts := make([]string, len(points))
+	for i, p := range points {
+		parts[i] = "(" + num(p.X) + ", " + num(p.Y) + ")"
+		if sized {
+			parts[i] += " " + num(p.Size)
+		}
+	}
+	return strings.Join(parts, " ")
+}