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/describe_test.go

@@ -0,0 +1,61 @@
+package chart
+
+import (
+	"math"
+	"testing"
+)
+
+func TestNumbersStayExact(t *testing.T) {
+	for v, want := range map[float64]string{
+		0: "0", 12: "12", 3.5: "3.5", -0.5: "-0.5", 0.01: "0.01", 1e-7: "0.0000001", 2e-7: "0.0000002",
+		1000001: "1000001", 12345: "12345", 0.1 + 0.2: "0.3",
+	} {
+		if got := num(v); got != want {
+			t.Errorf("num(%v) = %q, want %q", v, got, want)
+		}
+	}
+}
+
+func TestTicksKeepTheirStep(t *testing.T) {
+	for _, tc := range []struct {
+		v, step float64
+		want    string
+	}{
+		{15000, 5000, "15K"}, {12300, 100, "12.3K"}, {1.5e6, 1e5, "1.5M"}, {1000000.2, 0.2, "1000000.2"},
+		{0.6000000000000001, 0.2, "0.6"}, {2e-7, 1e-7, "0.0000002"}, {math.Copysign(0, -1), 1, "0"}, {40, 10, "40"},
+	} {
+		if got := tick(tc.v, tc.step); got != tc.want {
+			t.Errorf("tick(%v, %v) = %q, want %q", tc.v, tc.step, got, tc.want)
+		}
+	}
+}
+
+func TestDescriptionsCarryTheData(t *testing.T) {
+	for _, tc := range []struct{ got, want string }{
+		{read([]string{"A", "B"}, []float64{0.01, 0.02}), "A 0.01, B 0.02"},
+		{readSeries([]string{"X"}, []Series{{Name: "HP", Values: []float64{3}}, {Name: "MP", Values: []float64{1}}}), "HP: X 3; MP: X 1"},
+		{readSlices([]Slice{{Name: "Gold", Value: 52}}), "Gold 52"},
+		{where([]Point{{X: 1, Y: 3, Size: 9}}, true), "(1, 3) 9"},
+		{record([]int{1, 1, -1, 0}), "2-1-1: 1, 1, -1, 0"},
+		{ohlc([]Candle{{Label: "MON", Open: 10, High: 14, Low: 9, Close: 13}}), "MON 10, 14, 9, 13"},
+		{fives([]string{"D6"}, [][]float64{{1, 2, 3, 4, 5}}), "D6 1, 2, 3, 4, 5"},
+	} {
+		if tc.got != tc.want {
+			t.Errorf("got %q, want %q", tc.got, tc.want)
+		}
+	}
+}
+
+func TestFlatRangesGetDistinctTicks(t *testing.T) {
+	for _, r := range [][2]float64{{1e6, 1e6}, {40, 40}, {1e-5, 1e-5}, {-3, -3}, {1e6, 1e6 + 1}} {
+		from, step, n := ticks(r[0], r[1], 5)
+		top := from + float64(n-1)*step
+		if !(from < r[1] && r[0] < top) || tick(from, step) == tick(from+step, step) {
+			t.Errorf("%v ticks %v to %v by %v", r, from, top, step)
+		}
+	}
+	// All zeros keep zero as the floor, for bars.
+	if from, _, _ := ticks(0, 0, 5); from != 0 {
+		t.Errorf("flat zero ticks from %v", from)
+	}
+}