test(types,common): add unit tests, fix goimports formatting

- Add tests for types package: NewDataSet, NewCommit, Commit.String,
  Freq.Merge, DataSet operations
- Add tests for graph/common package: MinMax, ColorForFrequency,
  CreateGraph, SetColorScheme
- Fix goimports alignment in common.go and settings.go

graph/common/common_test.go

@@ -0,0 +1,125 @@
+package common
+
+import (
+	"testing"
+
+	sc "github.com/taigrr/simplecolorpalettes/simplecolor"
+)
+
+func TestMinMax(t *testing.T) {
+	tests := []struct {
+		name       string
+		input      []int
+		wantMin    int
+		wantMax    int
+	}{
+		{
+			name:    "normal values",
+			input:   []int{3, 1, 4, 1, 5, 9, 2, 6},
+			wantMin: 1,
+			wantMax: 9,
+		},
+		{
+			name:    "with zeros",
+			input:   []int{0, 3, 0, 7, 0, 2},
+			wantMin: 2,
+			wantMax: 7,
+		},
+		{
+			name:    "all zeros",
+			input:   []int{0, 0, 0},
+			wantMin: 0,
+			wantMax: 0,
+		},
+		{
+			name:    "single element",
+			input:   []int{5},
+			wantMin: 5,
+			wantMax: 5,
+		},
+		{
+			name:    "single zero",
+			input:   []int{0},
+			wantMin: 0,
+			wantMax: 0,
+		},
+		{
+			name:    "all same non-zero",
+			input:   []int{4, 4, 4},
+			wantMin: 4,
+			wantMax: 4,
+		},
+		{
+			name:    "empty slice",
+			input:   []int{},
+			wantMin: 0,
+			wantMax: 0,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			min, max := MinMax(tt.input)
+			if min != tt.wantMin {
+				t.Errorf("min: expected %d, got %d", tt.wantMin, min)
+			}
+			if max != tt.wantMax {
+				t.Errorf("max: expected %d, got %d", tt.wantMax, max)
+			}
+		})
+	}
+}
+
+func TestColorForFrequency(t *testing.T) {
+	// freq=0 should always return SimpleColor(0)
+	c := ColorForFrequency(0, 1, 10)
+	if c != sc.SimpleColor(0) {
+		t.Errorf("expected SimpleColor(0) for freq=0, got %v", c)
+	}
+
+	// Non-zero freq with small spread should return a valid color
+	c = ColorForFrequency(1, 1, 3)
+	if c == sc.SimpleColor(0) {
+		t.Error("expected non-zero color for freq=1 with small spread")
+	}
+
+	// Non-zero freq with large spread
+	c = ColorForFrequency(50, 1, 100)
+	if c == sc.SimpleColor(0) {
+		t.Error("expected non-zero color for freq=50 in range 1-100")
+	}
+
+	// Max frequency should return the last color
+	c = ColorForFrequency(100, 1, 100)
+	if c == sc.SimpleColor(0) {
+		t.Error("expected non-zero color for max frequency")
+	}
+
+	// Min frequency should return first real color
+	c = ColorForFrequency(1, 1, 100)
+	if c == sc.SimpleColor(0) {
+		t.Error("expected non-zero color for min frequency")
+	}
+}
+
+func TestCreateGraph(t *testing.T) {
+	buf := CreateGraph()
+	if buf.Len() != 0 {
+		t.Errorf("expected empty buffer, got %d bytes", buf.Len())
+	}
+}
+
+func TestSetColorScheme(t *testing.T) {
+	// Save original length
+	origLen := len(colorScheme)
+
+	// Reset after test
+	defer func() {
+		colorScheme = colorScheme[:origLen]
+	}()
+
+	// SetColorScheme should append colors
+	SetColorScheme(nil)
+	if len(colorScheme) != origLen {
+		t.Errorf("expected length %d after nil input, got %d", origLen, len(colorScheme))
+	}
+}