feat(tui): overhaul latency sparkline scaling, color, and layout

Replace misleading relative-only sparkline with dual-channel design:
bar height uses relative scaling (shows stability and anomalies),
color+brightness uses absolute thresholds (shows fast vs slow).

- Add brightness gradient within color bands (dim→bright as latency
  increases toward the next threshold)
- Pass row background through sparkline rendering so zebra stripes
  and selection highlights carry through ANSI sequences
- Cap sparkline width to 60 (matches maxHistoryLen) and column
  width to 62 to eliminate trailing dead space
- Quiet group sparkline: subtle dots for healthy, bold red for down
- Add braille subpixel canvas (ported from meridian) for future
  multi-row graph use

internal/tui/braille.go

@@ -0,0 +1,103 @@
+package tui
+
+// braillePlane is a subpixel canvas where each terminal cell maps to a 2×4
+// dot grid, rendered via Unicode braille (U+2800..U+28FF).
+type braillePlane struct {
+	wCells, hCells int
+	wDots, hDots   int
+	dots           []bool
+}
+
+func newBraillePlane(wCells, hCells int) *braillePlane {
+	wd, hd := wCells*2, hCells*4
+	return &braillePlane{
+		wCells: wCells, hCells: hCells,
+		wDots: wd, hDots: hd,
+		dots: make([]bool, wd*hd),
+	}
+}
+
+func (p *braillePlane) set(dx, dy int) {
+	if dx < 0 || dy < 0 || dx >= p.wDots || dy >= p.hDots {
+		return
+	}
+	p.dots[dy*p.wDots+dx] = true
+}
+
+// line draws a Bresenham line between two dot coordinates.
+func (p *braillePlane) line(x0, y0, x1, y1 int) {
+	dx := intAbs(x1 - x0)
+	sx := 1
+	if x0 >= x1 {
+		sx = -1
+	}
+	dy := -intAbs(y1 - y0)
+	sy := 1
+	if y0 >= y1 {
+		sy = -1
+	}
+	err := dx + dy
+	for {
+		p.set(x0, y0)
+		if x0 == x1 && y0 == y1 {
+			return
+		}
+		e2 := 2 * err
+		if e2 >= dy {
+			err += dy
+			x0 += sx
+		}
+		if e2 <= dx {
+			err += dx
+			y0 += sy
+		}
+	}
+}
+
+// fillBelow fills all dots below the topmost lit dot in each column,
+// producing an area-chart effect.
+func (p *braillePlane) fillBelow() {
+	for x := 0; x < p.wDots; x++ {
+		topY := -1
+		for y := 0; y < p.hDots; y++ {
+			if p.dots[y*p.wDots+x] {
+				topY = y
+				break
+			}
+		}
+		if topY >= 0 {
+			for y := topY + 1; y < p.hDots; y++ {
+				p.dots[y*p.wDots+x] = true
+			}
+		}
+	}
+}
+
+// cellMask builds the U+2800-relative bitmask for one terminal cell.
+func (p *braillePlane) cellMask(cx, cy int) byte {
+	type bit struct {
+		dx, dy int
+		m      byte
+	}
+	bits := [...]bit{
+		{0, 0, 0x01}, {0, 1, 0x02}, {0, 2, 0x04},
+		{1, 0, 0x08}, {1, 1, 0x10}, {1, 2, 0x20},
+		{0, 3, 0x40}, {1, 3, 0x80},
+	}
+	var mask byte
+	for _, b := range bits {
+		dx := cx*2 + b.dx
+		dy := cy*4 + b.dy
+		if dx >= 0 && dx < p.wDots && dy >= 0 && dy < p.hDots && p.dots[dy*p.wDots+dx] {
+			mask |= b.m
+		}
+	}
+	return mask
+}
+
+func intAbs(n int) int {
+	if n < 0 {
+		return -n
+	}
+	return n
+}

internal/tui/braille_test.go

@@ -0,0 +1,64 @@
+package tui
+
+import "testing"
+
+func TestBraillePlane_Set(t *testing.T) {
+	p := newBraillePlane(2, 1)
+	if p.wDots != 4 || p.hDots != 4 {
+		t.Fatalf("expected 4x4 dots, got %dx%d", p.wDots, p.hDots)
+	}
+	p.set(0, 0)
+	if !p.dots[0] {
+		t.Error("dot at (0,0) should be set")
+	}
+	p.set(-1, 0) // out of bounds, should not panic
+	p.set(0, 99) // out of bounds, should not panic
+}
+
+func TestBraillePlane_CellMask(t *testing.T) {
+	p := newBraillePlane(1, 1)
+	// Set bottom-left dot
+	p.set(0, 3)
+	mask := p.cellMask(0, 0)
+	if mask != 0x40 {
+		t.Errorf("bottom-left dot should be 0x40, got 0x%02x", mask)
+	}
+	// Set all dots
+	for y := 0; y < 4; y++ {
+		for x := 0; x < 2; x++ {
+			p.set(x, y)
+		}
+	}
+	mask = p.cellMask(0, 0)
+	if mask != 0xFF {
+		t.Errorf("all dots should be 0xFF, got 0x%02x", mask)
+	}
+}
+
+func TestBraillePlane_Line(t *testing.T) {
+	p := newBraillePlane(3, 1)
+	p.line(0, 2, 5, 2) // horizontal line
+	for x := 0; x <= 5; x++ {
+		if !p.dots[2*p.wDots+x] {
+			t.Errorf("dot at (%d, 2) should be set", x)
+		}
+	}
+}
+
+func TestBraillePlane_FillBelow(t *testing.T) {
+	p := newBraillePlane(1, 1)
+	p.set(0, 1) // set dot at row 1
+	p.fillBelow()
+	if !p.dots[1*p.wDots+0] {
+		t.Error("original dot should still be set")
+	}
+	if !p.dots[2*p.wDots+0] {
+		t.Error("row 2 should be filled")
+	}
+	if !p.dots[3*p.wDots+0] {
+		t.Error("row 3 should be filled")
+	}
+	if p.dots[0*p.wDots+0] {
+		t.Error("row 0 above the dot should not be filled")
+	}
+}

internal/tui/sparkline.go

@@ -1,15 +1,63 @@
 package tui
 
 import (
+	"fmt"
 	"strings"
 	"time"
+
+	"github.com/charmbracelet/lipgloss"
 )
 
 var sparkChars = []rune{'▁', '▂', '▃', '▄', '▅', '▆', '▇', '█'}
 
-func latencySparkline(latencies []time.Duration, statuses []bool, width int) string {
+func parseHex(hex string) (r, g, b uint8) {
+	if len(hex) == 7 && hex[0] == '#' {
+		fmt.Sscanf(hex[1:], "%02x%02x%02x", &r, &g, &b)
+	}
+	return
+}
+
+func dimColor(hex string, brightness float64) lipgloss.Color {
+	r, g, b := parseHex(hex)
+	f := 0.3 + brightness*0.7
+	return lipgloss.Color(fmt.Sprintf("#%02x%02x%02x",
+		uint8(float64(r)*f),
+		uint8(float64(g)*f),
+		uint8(float64(b)*f),
+	))
+}
+
+func withBg(s lipgloss.Style, bg lipgloss.Color) lipgloss.Style {
+	if bg != "" {
+		return s.Background(bg)
+	}
+	return s
+}
+
+func latencyStyle(ms int64, bg lipgloss.Color) lipgloss.Style {
+	var hex string
+	var t float64
+	switch {
+	case ms < 200:
+		hex = sparkSuccess
+		t = float64(ms) / 200
+	case ms < 500:
+		hex = sparkWarning
+		t = float64(ms-200) / 300
+	default:
+		hex = sparkDanger
+		t = float64(ms-500) / 1500
+		if t > 1 {
+			t = 1
+		}
+	}
+	s := lipgloss.NewStyle().Foreground(dimColor(hex, t))
+	return withBg(s, bg)
+}
+
+func latencySparkline(latencies []time.Duration, statuses []bool, width int, bg lipgloss.Color) string {
 	if len(latencies) == 0 {
-		return subtleStyle.Render(strings.Repeat("·", width))
+		return withBg(subtleStyle, bg).Render(strings.Repeat("·", width))
 	}
 
 	samples := latencies
@@ -30,12 +78,12 @@ func latencySparkline(latencies []time.Duration, statuses []bool, width int) str
 			maxL = l
 		}
 	}
+	spread := maxL - minL
 
 	var sb strings.Builder
 	if remaining := width - len(samples); remaining > 0 {
-		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
+		sb.WriteString(withBg(subtleStyle, bg).Render(strings.Repeat("·", remaining)))
 	}
-	spread := maxL - minL
 	for i, l := range samples {
 		idx := 0
 		if spread > 0 {
@@ -47,24 +95,17 @@ func latencySparkline(latencies []time.Duration, statuses []bool, width int) str
 		ch := string(sparkChars[idx])
 		isDown := i < len(sampledStatuses) && !sampledStatuses[i]
 		if isDown {
-			sb.WriteString(dangerStyle.Render(ch))
+			sb.WriteString(withBg(dangerStyle, bg).Render(ch))
 		} else {
-			ms := l.Milliseconds()
-			if ms < 200 {
-				sb.WriteString(specialStyle.Render(ch))
-			} else if ms < 500 {
-				sb.WriteString(warnStyle.Render(ch))
-			} else {
-				sb.WriteString(dangerStyle.Render(ch))
-			}
+			sb.WriteString(latencyStyle(l.Milliseconds(), bg).Render(ch))
 		}
 	}
 	return sb.String()
 }
 
-func heartbeatSparkline(statuses []bool, width int) string {
+func heartbeatSparkline(statuses []bool, width int, bg lipgloss.Color) string {
 	if len(statuses) == 0 {
-		return subtleStyle.Render(strings.Repeat("·", width))
+		return withBg(subtleStyle, bg).Render(strings.Repeat("·", width))
 	}
 
 	samples := statuses
@@ -74,19 +115,19 @@ func heartbeatSparkline(statuses []bool, width int) string {
 
 	var sb strings.Builder
 	if remaining := width - len(samples); remaining > 0 {
-		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
+		sb.WriteString(withBg(subtleStyle, bg).Render(strings.Repeat("·", remaining)))
 	}
 	for _, up := range samples {
 		if up {
-			sb.WriteString(specialStyle.Render("▁"))
+			sb.WriteString(withBg(specialStyle, bg).Render("▁"))
 		} else {
-			sb.WriteString(dangerStyle.Render("█"))
+			sb.WriteString(withBg(dangerStyle, bg).Render("█"))
 		}
 	}
 	return sb.String()
 }
 
-func (m Model) groupSparkline(groupID int, width int) string {
+func (m Model) groupSparkline(groupID int, width int, bg lipgloss.Color) string {
 	allSites := m.engine.GetAllSites()
 	var childStatuses [][]bool
 	for _, s := range allSites {
@@ -99,7 +140,7 @@ func (m Model) groupSparkline(groupID int, width int) string {
 	}
 
 	if len(childStatuses) == 0 {
-		return subtleStyle.Render(strings.Repeat("·", width))
+		return withBg(subtleStyle, bg).Render(strings.Repeat("·", width))
 	}
 
 	maxLen := 0
@@ -127,13 +168,13 @@ func (m Model) groupSparkline(groupID int, width int) string {
 
 	var sb strings.Builder
 	if remaining := width - len(aggregated); remaining > 0 {
-		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
+		sb.WriteString(withBg(subtleStyle, bg).Render(strings.Repeat("·", remaining)))
 	}
 	for _, up := range aggregated {
 		if up {
-			sb.WriteString(specialStyle.Render("•"))
+			sb.WriteString(withBg(subtleStyle, bg).Render("·"))
 		} else {
-			sb.WriteString(dangerStyle.Render("•"))
+			sb.WriteString(withBg(dangerStyle, bg).Render("•"))
 		}
 	}
 	return sb.String()

internal/tui/sparkline_test.go

@@ -4,10 +4,11 @@ import (
 	"strings"
 	"testing"
 	"time"
+	"unicode/utf8"
 )
 
 func TestLatencySparkline_Empty(t *testing.T) {
-	got := latencySparkline(nil, nil, 10)
+	got := latencySparkline(nil, nil, 10, "")
 	if !strings.Contains(got, "··········") {
 		t.Errorf("empty sparkline should be dots, got %q", got)
 	}
@@ -16,10 +17,13 @@ func TestLatencySparkline_Empty(t *testing.T) {
 func TestLatencySparkline_SingleValue(t *testing.T) {
 	latencies := []time.Duration{100 * time.Millisecond}
 	statuses := []bool{true}
-	got := latencySparkline(latencies, statuses, 5)
+	got := latencySparkline(latencies, statuses, 5, "")
 	if len(got) == 0 {
 		t.Error("sparkline should not be empty")
 	}
+	if !strings.Contains(got, "····") {
+		t.Errorf("single value with width=5 should have 4 dot padding, got %q", got)
+	}
 }
 
 func TestLatencySparkline_WidthTruncation(t *testing.T) {
@@ -29,14 +33,92 @@ func TestLatencySparkline_WidthTruncation(t *testing.T) {
 		latencies[i] = time.Duration(i*50) * time.Millisecond
 		statuses[i] = true
 	}
-	got := latencySparkline(latencies, statuses, 5)
+	got := latencySparkline(latencies, statuses, 5, "")
 	if len(got) == 0 {
 		t.Error("sparkline should not be empty")
 	}
+	if strings.Contains(got, "·") {
+		t.Errorf("20 samples in width=5 should have no padding, got %q", got)
+	}
+}
+
+func TestLatencySparkline_RelativeHeight(t *testing.T) {
+	latencies := []time.Duration{10 * time.Millisecond, 50 * time.Millisecond, 10 * time.Millisecond}
+	statuses := []bool{true, true, true}
+	out := stripANSI(latencySparkline(latencies, statuses, 3, ""))
+	runes := []rune(out)
+	if len(runes) < 3 {
+		t.Fatalf("expected 3 runes, got %d", len(runes))
+	}
+	if runes[0] == runes[1] {
+		t.Errorf("min and max should have different bar heights, got %c %c %c", runes[0], runes[1], runes[2])
+	}
+}
+
+func TestLatencyStyle_BandsProduceDifferentColors(t *testing.T) {
+	sparkSuccess = "#00ff00"
+	sparkWarning = "#ffff00"
+	sparkDanger = "#ff0000"
+	defer func() {
+		sparkSuccess = ""
+		sparkWarning = ""
+		sparkDanger = ""
+	}()
+
+	green := latencyStyle(50, "")
+	yellow := latencyStyle(300, "")
+	red := latencyStyle(800, "")
+
+	gfg := green.GetForeground()
+	yfg := yellow.GetForeground()
+	rfg := red.GetForeground()
+
+	if gfg == yfg || yfg == rfg || gfg == rfg {
+		t.Errorf("bands should produce distinct foreground colors: green=%v yellow=%v red=%v", gfg, yfg, rfg)
+	}
+}
+
+func TestLatencyStyle_BrightnessVariesWithinBand(t *testing.T) {
+	sparkSuccess = "#00ff00"
+	defer func() { sparkSuccess = "" }()
+
+	dim := latencyStyle(10, "")
+	bright := latencyStyle(190, "")
+
+	if dim.GetForeground() == bright.GetForeground() {
+		t.Error("10ms and 190ms should have different brightness within green band")
+	}
+}
+
+func TestLatencySparkline_OutputWidth(t *testing.T) {
+	latencies := []time.Duration{100 * time.Millisecond, 200 * time.Millisecond, 300 * time.Millisecond}
+	statuses := []bool{true, true, true}
+	got := latencySparkline(latencies, statuses, 5, "")
+	count := utf8.RuneCountInString(stripANSI(got))
+	if count != 5 {
+		t.Errorf("expected 5 rune-width output, got %d from %q", count, got)
+	}
+}
+
+func stripANSI(s string) string {
+	var out strings.Builder
+	i := 0
+	for i < len(s) {
+		if s[i] == '\x1b' {
+			for i < len(s) && s[i] != 'm' {
+				i++
+			}
+			i++
+			continue
+		}
+		out.WriteByte(s[i])
+		i++
+	}
+	return out.String()
 }
 
 func TestHeartbeatSparkline_Empty(t *testing.T) {
-	got := heartbeatSparkline(nil, 10)
+	got := heartbeatSparkline(nil, 10, "")
 	if !strings.Contains(got, "··········") {
 		t.Errorf("empty heartbeat should be dots, got %q", got)
 	}
@@ -44,7 +126,7 @@ func TestHeartbeatSparkline_Empty(t *testing.T) {
 
 func TestHeartbeatSparkline_Mixed(t *testing.T) {
 	statuses := []bool{true, false, true, true, false}
-	got := heartbeatSparkline(statuses, 5)
+	got := heartbeatSparkline(statuses, 5, "")
 	if len(got) == 0 {
 		t.Error("heartbeat sparkline should not be empty")
 	}
@@ -52,7 +134,7 @@ func TestHeartbeatSparkline_Mixed(t *testing.T) {
 
 func TestHeartbeatSparkline_PaddedWidth(t *testing.T) {
 	statuses := []bool{true, true}
-	got := heartbeatSparkline(statuses, 5)
+	got := heartbeatSparkline(statuses, 5, "")
 	if !strings.Contains(got, "···") {
 		t.Errorf("should have dot padding for width > data, got %q", got)
 	}

internal/tui/tab_sites.go

@@ -86,6 +86,9 @@ func (m Model) computeLayout() tableLayout {
 	if sparkW < 15 {
 		sparkW = 15
 	}
+	if sparkW > 62 {
+		sparkW = 62
+	}
 
 	widths[1] = nameW
 	widths[6] = sparkW
@@ -110,6 +113,9 @@ func (m Model) viewSitesTab() string {
 	if sparkWidth < 8 {
 		sparkWidth = 8
 	}
+	if sparkWidth > 60 {
+		sparkWidth = 60
+	}
 
 	var groupRows map[int]bool
 	return m.renderTable(
@@ -120,6 +126,13 @@ func (m Model) viewSitesTab() string {
 			var rows [][]string
 			for i := start; i < end; i++ {
 				site := m.sites[i]
+				rowIdx := i - start
+				var rowBg lipgloss.Color
+				if i == m.cursor {
+					rowBg = m.theme.SelectedBg
+				} else if rowIdx%2 == 1 {
+					rowBg = m.theme.ZebraBg
+				}
 
 				if site.Type == "group" {
 					groupRows[i-start] = true
@@ -131,7 +144,7 @@ func (m Model) viewSitesTab() string {
 						fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)),
 						subtleStyle.Render("—"),
 						m.groupUptime(site.ID),
-						m.groupSparkline(site.ID, sparkWidth),
+						m.groupSparkline(site.ID, sparkWidth, rowBg),
 						subtleStyle.Render("-"),
 						subtleStyle.Render("—"),
 					})
@@ -166,9 +179,9 @@ func (m Model) viewSitesTab() string {
 				hist, _ := m.engine.GetHistory(site.ID)
 				var spark string
 				if site.Type == "push" {
-					spark = heartbeatSparkline(hist.Statuses, sparkWidth)
+					spark = heartbeatSparkline(hist.Statuses, sparkWidth, rowBg)
 				} else {
-					spark = latencySparkline(hist.Latencies, hist.Statuses, sparkWidth)
+					spark = latencySparkline(hist.Latencies, hist.Statuses, sparkWidth, rowBg)
 				}
 
 				rows = append(rows, []string{

internal/tui/tui.go

@@ -25,6 +25,10 @@ var (
 	titleStyle   lipgloss.Style
 	activeTab    lipgloss.Style
 	inactiveTab  lipgloss.Style
+
+	sparkSuccess string
+	sparkWarning string
+	sparkDanger  string
 )
 
 func applyTheme(t Theme) {
@@ -33,6 +37,11 @@ func applyTheme(t Theme) {
 	warnStyle = lipgloss.NewStyle().Foreground(t.Warning)
 	staleStyle = lipgloss.NewStyle().Foreground(t.Stale)
 	dangerStyle = lipgloss.NewStyle().Foreground(t.Danger)
+
+	sparkSuccess = string(t.Success)
+	sparkWarning = string(t.Warning)
+	sparkDanger = string(t.Danger)
+
 	titleStyle = lipgloss.NewStyle().Foreground(t.Accent).Bold(true)
 	activeTab = lipgloss.NewStyle().Border(lipgloss.NormalBorder(), false, false, true, false).BorderForeground(t.Accent).Foreground(t.Accent).Bold(true).Padding(0, 1)
 	inactiveTab = lipgloss.NewStyle().Padding(0, 1).Foreground(t.Muted)

internal/tui/view_detail.go

@@ -203,7 +203,7 @@ func (m Model) viewDetailPanel() string {
 	b.WriteString(m.divider() + "\n")
 	const sparkWidth = 40
 	if site.Type == "push" {
-		b.WriteString("  " + heartbeatSparkline(hist.Statuses, sparkWidth))
+		b.WriteString("  " + heartbeatSparkline(hist.Statuses, sparkWidth, ""))
 		if len(hist.Statuses) > 0 {
 			up := 0
 			for _, s := range hist.Statuses {
@@ -216,7 +216,7 @@ func (m Model) viewDetailPanel() string {
 				up, len(hist.Statuses))
 		}
 	} else {
-		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth))
+		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth, ""))
 		var minL, maxL, total time.Duration
 		count := 0
 		for i, l := range hist.Latencies {

internal/tui/view_history.go

@@ -47,6 +47,8 @@ func computeHistoryStats(changes []models.StateChange) historyStats {
 	return s
 }
 
+var stateChangeChars = []rune{'▁', '▂', '▃', '▄', '▅', '▆', '▇', '█'}
+
 func stateChangeSparkline(changes []models.StateChange, width int) string {
 	if len(changes) < 2 || width < 4 {
 		return ""
@@ -91,7 +93,7 @@ func stateChangeSparkline(changes []models.StateChange, width int) string {
 		if idx > 7 {
 			idx = 7
 		}
-		ch := string(sparkChars[idx])
+		ch := string(stateChangeChars[idx])
 		switch {
 		case v >= 3:
 			sb.WriteString(dangerStyle.Render(ch))