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/chart_test.go

@@ -0,0 +1,102 @@
+package chart_test
+
+import (
+	"image"
+	"testing"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type clock struct{ t time.Time }
+
+func (c *clock) now() time.Time { return c.t }
+
+func newClock() *clock { return &clock{time.Date(2026, 10, 12, 9, 0, 0, 0, time.UTC)} }
+
+// view builds charts on a fake clock.
+func view(w, h int, clk *clock, build func(c *ui.Context)) *ui.Tester {
+	return ui.NewTester(func(c *ui.Context) {
+		pixel.Use(c, pixel.Settings{Palette: pixel.SpringDay, Now: clk.now})
+		build(c)
+	}, w, h)
+}
+
+func find(t *testing.T, tt *ui.Tester, label string) ui.Rect {
+	t.Helper()
+	r, ok := tt.Find(label)
+	if !ok {
+		t.Fatalf("no %q in %q", label, tt.Texts())
+	}
+	return r
+}
+
+// at is the slot of dot x, y.
+func at(t *testing.T, img *image.RGBA, x, y int) pixel.Slot {
+	t.Helper()
+	c := img.RGBAAt(2*x, 2*y)
+	for s, pc := range pixel.SpringDay.Colors {
+		if pc.R == c.R && pc.G == c.G && pc.B == c.B {
+			return pixel.Slot(s)
+		}
+	}
+	t.Fatalf("dot %d,%d is %v, off the palette", x, y, c)
+	return 0
+}
+
+// count tallies dots of s in a dot box.
+func count(t *testing.T, img *image.RGBA, x, y, w, h int, s pixel.Slot) int {
+	t.Helper()
+	n := 0
+	for j := y; j < y+h; j++ {
+		for i := x; i < x+w; i++ {
+			if at(t, img, i, j) == s {
+				n++
+			}
+		}
+	}
+	return n
+}
+
+// keeps fails on off-palette colors or split dots.
+func keeps(t *testing.T, img *image.RGBA) {
+	t.Helper()
+	b := img.Bounds()
+	for y := 0; y+1 < b.Dy(); y += 2 {
+		for x := 0; x+1 < b.Dx(); x += 2 {
+			at(t, img, x/2, y/2)
+			c := img.RGBAAt(x, y)
+			if img.RGBAAt(x+1, y) != c || img.RGBAAt(x, y+1) != c || img.RGBAAt(x+1, y+1) != c {
+				t.Fatalf("dot at %d,%d is split", x/2, y/2)
+			}
+		}
+	}
+}
+
+// panics fails unless fn panics.
+func panics(t *testing.T, what string, fn func()) {
+	t.Helper()
+	defer func() {
+		if recover() == nil {
+			t.Fatalf("%s did not panic", what)
+		}
+	}()
+	fn()
+}
+
+// column is the top row and count of s.
+func column(t *testing.T, img *image.RGBA, x, h int, s pixel.Slot) (top, n int) {
+	t.Helper()
+	top = -1
+	for y := range h {
+		if at(t, img, x, y) == s {
+			if top < 0 {
+				top = y
+			}
+			n++
+		}
+	}
+	return top, n
+}