fix(plot): keep extreme finite ranges drawable

Assisted-by: GLM 5.3 Flash

plot/plot_test.go

@@ -103,6 +103,39 @@ func TestWriteSVGErrors(t *testing.T) {
 	}
 }
 
+// TestWriteSVGExtremeFiniteValuesStayFinite pins the renderer against
+// the extreme-but-finite corner: data, or an explicit axis range, whose
+// span overflows float64 must not leak NaN or Inf coordinates into the
+// drawing, because no renderer displays them and the non-finite
+// refusal at the door guarantees every input point is finite.
+func TestWriteSVGExtremeFiniteValuesStayFinite(t *testing.T) {
+	dir := t.TempDir()
+	padded := sampleChart()
+	padded.XRange = [2]float64{-1e308, 1e308}
+	padded.YRange = [2]float64{-1e308, 1e308}
+	charts := []struct {
+		name  string
+		chart Chart
+	}{
+		{"wide-data.svg", Chart{Series: []Series{{Name: "wide",
+			Points: []Point{{X: 0, Y: -1e308}, {X: 1, Y: 1e308}}}}}},
+		{"wide-range.svg", padded},
+	}
+	for _, tc := range charts {
+		path := filepath.Join(dir, tc.name)
+		if err := tc.chart.WriteSVG(path); err != nil {
+			t.Fatalf("%s: %v", tc.name, err)
+		}
+		body, err := os.ReadFile(path)
+		if err != nil {
+			t.Fatalf("%s: %v", tc.name, err)
+		}
+		if s := string(body); strings.Contains(s, "NaN") || strings.Contains(s, "Inf") {
+			t.Fatalf("%s: the rendering leaked a non-finite coordinate: %s", tc.name, s)
+		}
+	}
+}
+
 func mustFromFloats(t *testing.T, values []float64, shape ...int) *core.Array {
 	t.Helper()
 	a, err := core.FromFloats(values, shape...)