3 files changed, 186 insertions, 2 deletions
diff --git a/internal/tui/dashboard/histogram.go b/internal/tui/dashboard/histogram.go
new file mode 100644
index 0000000..a95159a
--- /dev/null
+++ b/internal/tui/dashboard/histogram.go
@@ -0,0 +1,108 @@
+package dashboard
+
+import (
+	"fmt"
+	"ior/internal/statsengine"
+	"ior/internal/tui"
+	"math"
+	"strconv"
+	"strings"
+)
+
+func renderLatencyTab(snap *statsengine.Snapshot, width, height int) string {
+	if snap == nil {
+		return tui.PanelStyle.Render("Latency: waiting for stats...")
+	}
+
+	hist := renderHistogram(snap.LatencyHistogram, "Latency Histogram", width, height)
+	spark := tui.PanelStyle.Render("Latency sparkline: " + renderSparkline(snap.LatencySeriesNs(), sparklineWidth(width)))
+	return strings.Join([]string{hist, spark}, "\n")
+}
+
+func renderGapsTab(snap *statsengine.Snapshot, width, height int) string {
+	if snap == nil {
+		return tui.PanelStyle.Render("Gaps: waiting for stats...")
+	}
+
+	hist := renderHistogram(snap.GapHistogram, "Gap Histogram", width, height)
+	spark := tui.PanelStyle.Render("Gap sparkline: " + renderSparkline(snap.GapSeriesNs(), sparklineWidth(width)))
+	return strings.Join([]string{hist, spark}, "\n")
+}
+
+func renderHistogram(hist statsengine.HistogramSnapshot, title string, width, height int) string {
+	buckets := hist.Buckets()
+	if len(buckets) == 0 {
+		return tui.PanelStyle.Render(title + ": no data")
+	}
+
+	if width <= 0 {
+		width = 80
+	}
+
+	if height > 0 {
+		maxRows := height - 3
+		if maxRows < 1 {
+			maxRows = 1
+		}
+		if len(buckets) > maxRows {
+			buckets = buckets[:maxRows]
+		}
+	}
+
+	maxCount := uint64(0)
+	labelWidth := 0
+	countWidth := len(strconv.FormatUint(hist.Total, 10))
+	for _, bucket := range buckets {
+		if bucket.Count > maxCount {
+			maxCount = bucket.Count
+		}
+		if len(bucket.Label) > labelWidth {
+			labelWidth = len(bucket.Label)
+		}
+		if digits := len(strconv.FormatUint(bucket.Count, 10)); digits > countWidth {
+			countWidth = digits
+		}
+	}
+
+	barWidth := width - labelWidth - countWidth - 10
+	if barWidth < 8 {
+		barWidth = 8
+	}
+
+	lines := make([]string, 0, len(buckets)+2)
+	lines = append(lines, fmt.Sprintf("%s (total=%d)", title, hist.Total))
+	for _, bucket := range buckets {
+		bar := renderHistogramBar(bucket.Count, maxCount, barWidth)
+		lines = append(lines, fmt.Sprintf("%-*s | %-*s %*d", labelWidth, bucket.Label, barWidth, bar, countWidth, bucket.Count))
+	}
+	lines = append(lines, "Scale: █▓▒░")
+
+	return tui.PanelStyle.Render(strings.Join(lines, "\n"))
+}
+
+func renderHistogramBar(count, maxCount uint64, width int) string {
+	if count == 0 || maxCount == 0 || width <= 0 {
+		return ""
+	}
+
+	ratio := float64(count) / float64(maxCount)
+	length := int(math.Round(ratio * float64(width)))
+	if length < 1 {
+		length = 1
+	}
+	if length > width {
+		length = width
+	}
+
+	char := '░'
+	switch {
+	case ratio >= 0.75:
+		char = '█'
+	case ratio >= 0.5:
+		char = '▓'
+	case ratio >= 0.25:
+		char = '▒'
+	}
+
+	return strings.Repeat(string(char), length)
+}
diff --git a/internal/tui/dashboard/histogram_test.go b/internal/tui/dashboard/histogram_test.go
new file mode 100644
index 0000000..9da1c47
--- /dev/null
+++ b/internal/tui/dashboard/histogram_test.go
@@ -0,0 +1,76 @@
+package dashboard
+
+import (
+	"strings"
+	"testing"
+
+	"ior/internal/statsengine"
+)
+
+func TestRenderHistogramNoBuckets(t *testing.T) {
+	out := renderHistogram(statsengine.HistogramSnapshot{}, "Latency Histogram", 80, 20)
+	if !strings.Contains(out, "no data") {
+		t.Fatalf("expected no data placeholder, got %q", out)
+	}
+}
+
+func TestRenderHistogramIncludesLabelsCountsAndScale(t *testing.T) {
+	hist := statsengine.NewHistogramSnapshot(13, []statsengine.HistogramBucketSnapshot{
+		{Label: "[0,1us)", Count: 1},
+		{Label: "[1us,10us)", Count: 3},
+		{Label: "[10us,100us)", Count: 9},
+	})
+
+	out := renderHistogram(hist, "Latency Histogram", 100, 20)
+	for _, token := range []string{"Latency Histogram", "[0,1us)", "[1us,10us)", "[10us,100us)", "9", "Scale: █▓▒░"} {
+		if !strings.Contains(out, token) {
+			t.Fatalf("expected token %q in histogram output", token)
+		}
+	}
+	if !strings.Contains(out, "█") {
+		t.Fatalf("expected high-intensity bar rune in histogram output")
+	}
+}
+
+func TestRenderLatencyAndGapsTabIncludeSparkline(t *testing.T) {
+	snap := statsengine.NewSnapshot(
+		[]float64{10, 20, 15, 30},
+		[]float64{2, 4, 3, 5},
+		nil,
+		nil,
+		nil,
+		nil,
+		statsengine.NewHistogramSnapshot(2, []statsengine.HistogramBucketSnapshot{
+			{Label: "[0,1us)", Count: 2},
+		}),
+		statsengine.NewHistogramSnapshot(1, []statsengine.HistogramBucketSnapshot{
+			{Label: "[1us,10us)", Count: 1},
+		}),
+	)
+
+	lat := renderLatencyTab(&snap, 100, 24)
+	if !strings.Contains(lat, "Latency Histogram") || !strings.Contains(lat, "Latency sparkline:") {
+		t.Fatalf("latency tab missing expected sections: %q", lat)
+	}
+
+	gap := renderGapsTab(&snap, 100, 24)
+	if !strings.Contains(gap, "Gap Histogram") || !strings.Contains(gap, "Gap sparkline:") {
+		t.Fatalf("gaps tab missing expected sections: %q", gap)
+	}
+}
+
+func TestRenderHistogramTruncatesForSmallHeight(t *testing.T) {
+	hist := statsengine.NewHistogramSnapshot(3, []statsengine.HistogramBucketSnapshot{
+		{Label: "[0,1us)", Count: 1},
+		{Label: "[1us,10us)", Count: 1},
+		{Label: "[10us,100us)", Count: 1},
+	})
+
+	out := renderHistogram(hist, "Latency Histogram", 100, 3)
+	if !strings.Contains(out, "[0,1us)") {
+		t.Fatalf("expected first bucket in output: %q", out)
+	}
+	if strings.Contains(out, "[1us,10us)") || strings.Contains(out, "[10us,100us)") {
+		t.Fatalf("expected histogram rows to be truncated for small height: %q", out)
+	}
+}
diff --git a/internal/tui/dashboard/model.go b/internal/tui/dashboard/model.go
index 44a6da7..454e8ab 100644
--- a/internal/tui/dashboard/model.go
+++ b/internal/tui/dashboard/model.go
@@ -169,9 +169,9 @@ func renderActiveTab(tab Tab, snap *statsengine.Snapshot, width, height, syscall
 	case TabProcesses:
 		return renderProcessesWithOffset(snap, width, height, processesOffset)
 	case TabLatency:
-		return tui.PanelStyle.Render("Latency histogram")
+		return renderLatencyTab(snap, width, height)
 	case TabGaps:
-		return tui.PanelStyle.Render("Gap histogram")
+		return renderGapsTab(snap, width, height)
 	default:
 		return tui.PanelStyle.Render("Unknown tab")
 	}