diff --git a/main.go b/main.go
index a0aa3ab..134c5f5 100644
--- a/main.go
+++ b/main.go
@@ -25,6 +25,8 @@ type store struct {
 	history   []sample
 }
 
+const maxHistoryPoints = 10000
+
 type sample struct {
 	at    time.Time
 	value int
@@ -42,6 +44,11 @@ func (s *store) set(v int) {
 	now := time.Now()
 	s.updatedAt = now
 	s.history = append(s.history, sample{at: now, value: v})
+	s.trimHistoryLocked(now)
+	s.mu.Unlock()
+}
+
+func (s *store) trimHistoryLocked(now time.Time) {
 	cutoff := now.Add(-12 * time.Hour)
 	keep := s.history[:0]
 	for _, point := range s.history {
@@ -49,8 +56,10 @@ func (s *store) set(v int) {
 			keep = append(keep, point)
 		}
 	}
+	if len(keep) > maxHistoryPoints {
+		keep = keep[len(keep)-maxHistoryPoints:]
+	}
 	s.history = keep
-	s.mu.Unlock()
 }
 
 func (s *store) snapshot() (int, time.Time, []sample) {
@@ -63,6 +72,7 @@ func (s *store) snapshot() (int, time.Time, []sample) {
 func main() {
 	secret := os.Getenv("WRITE_SECRET")
 	addr := os.Getenv("ADDR")
+	chartMax := 0
 	if cfg, err := loadConfig("config.json"); err != nil {
 		if !errors.Is(err, os.ErrNotExist) {
 			log.Fatalf("load config: %v", err)
@@ -74,6 +84,9 @@ func main() {
 		if addr == "" {
 			addr = cfg.Addr
 		}
+		if cfg.ChartMaxWatts > 0 {
+			chartMax = cfg.ChartMaxWatts
+		}
 	}
 	if secret == "" {
 		log.Fatal("WRITE_SECRET is required")
@@ -83,8 +96,9 @@ func main() {
 	}
 
 	app := &server{
-		secret: secret,
-		store:  &store{},
+		secret:   secret,
+		chartMax: chartMax,
+		store:    &store{},
 	}
 
 	log.Printf("listening on %s", addr)
@@ -92,13 +106,15 @@ func main() {
 }
 
 type server struct {
-	secret string
-	store  *store
+	secret   string
+	chartMax int
+	store    *store
 }
 
 type config struct {
-	WriteSecret string `json:"write_secret"`
-	Addr        string `json:"addr"`
+	WriteSecret   string `json:"write_secret"`
+	Addr          string `json:"addr"`
+	ChartMaxWatts int    `json:"chart_max_watts"`
 }
 
 func loadConfig(path string) (config, error) {
@@ -143,17 +159,17 @@ func (s *server) handleIndex(w http.ResponseWriter, r *http.Request) {
 	if err := page.Execute(w, map[string]any{
 		"Watts":        watts,
 		"UpdatedAt":    updatedLabel,
-		"HistoryChart": buildHistoryChart(history),
+		"HistoryChart": buildHistoryChart(history, s.chartMax),
 	}); err != nil {
 		http.Error(w, "render error", http.StatusInternalServerError)
 	}
 }
 
-func buildHistoryChart(history []sample) template.HTML {
+func buildHistoryChart(history []sample, maxWatts int) template.HTML {
 	const (
 		width  = 640
 		height = 220
-		padL   = 52
+		padL   = 72
 		padR   = 18
 		padT   = 18
 		padB   = 42
@@ -164,10 +180,13 @@ func buildHistoryChart(history []sample) template.HTML {
 	}
 
 	minV, maxV := 0, history[0].value
+	if maxWatts > 0 {
+		maxV = maxWatts
+	}
 	start := history[0].at
 	end := history[len(history)-1].at
 	for _, point := range history[1:] {
-		if point.value > maxV {
+		if maxWatts <= 0 && point.value > maxV {
 			maxV = point.value
 		}
 		if point.at.Before(start) {
@@ -193,6 +212,12 @@ func buildHistoryChart(history []sample) template.HTML {
 	for _, point := range history {
 		x := float64(padL) + plotW*point.at.Sub(start).Seconds()/span.Seconds()
 		y := float64(height-padB) - plotH*(float64(point.value-minV)/float64(maxV-minV))
+		if y < float64(padT) {
+			y = float64(padT)
+		}
+		if y > float64(height-padB) {
+			y = float64(height - padB)
+		}
 		points = append(points, fmt.Sprintf("%.1f,%.1f", x, y))
 		lastX, lastY = x, y
 	}
diff --git a/main_test.go b/main_test.go
index 7eaddb1..a8f4193 100644
--- a/main_test.go
+++ b/main_test.go
@@ -66,7 +66,7 @@ func TestHandleWriteStoresValue(t *testing.T) {
 
 func TestLoadConfig(t *testing.T) {
 	path := t.TempDir() + "/config.json"
-	if err := os.WriteFile(path, []byte(`{"write_secret":"secret","addr":":9090"}`), 0o600); err != nil {
+	if err := os.WriteFile(path, []byte(`{"write_secret":"secret","addr":":9090","chart_max_watts":750}`), 0o600); err != nil {
 		t.Fatalf("write config: %v", err)
 	}
 
@@ -80,10 +80,13 @@ func TestLoadConfig(t *testing.T) {
 	if cfg.Addr != ":9090" {
 		t.Fatalf("addr = %q, want %q", cfg.Addr, ":9090")
 	}
+	if cfg.ChartMaxWatts != 750 {
+		t.Fatalf("chart_max_watts = %d, want %d", cfg.ChartMaxWatts, 750)
+	}
 }
 
 func TestBuildHistoryChartEmpty(t *testing.T) {
-	chart := buildHistoryChart(nil)
+	chart := buildHistoryChart(nil, 0)
 	if !strings.Contains(string(chart), "No power history yet") {
 		t.Fatalf("empty chart did not contain fallback text: %s", string(chart))
 	}
@@ -96,7 +99,7 @@ func TestBuildHistoryChartRendersPolyline(t *testing.T) {
 		{at: time.Date(2026, 4, 20, 12, 0, 0, 0, time.UTC), value: 150},
 	}
 
-	chart := buildHistoryChart(history)
+	chart := buildHistoryChart(history, 0)
 	if !strings.Contains(string(chart), "<polyline") {
 		t.Fatalf("chart did not contain polyline: %s", string(chart))
 	}
@@ -109,3 +112,33 @@ func TestBuildHistoryChartRendersPolyline(t *testing.T) {
 		t.Fatalf("chart did not include zero baseline: %s", string(chart))
 	}
 }
+
+func TestBuildHistoryChartUsesConfiguredMax(t *testing.T) {
+	history := []sample{{at: time.Date(2026, 4, 20, 12, 0, 0, 0, time.UTC), value: 250}}
+	chart := buildHistoryChart(history, 1000)
+	if !strings.Contains(string(chart), "1000 W") {
+		t.Fatalf("chart did not include configured max: %s", string(chart))
+	}
+	if strings.Contains(string(chart), "250 W") {
+		t.Fatalf("chart unexpectedly used sample max instead of configured max: %s", string(chart))
+	}
+}
+
+func TestStoreTrimsHistoryToMaxPoints(t *testing.T) {
+	s := &store{}
+	base := time.Date(2026, 4, 20, 0, 0, 0, 0, time.UTC)
+	s.mu.Lock()
+	for i := 0; i < maxHistoryPoints+25; i++ {
+		s.history = append(s.history, sample{at: base.Add(time.Duration(i) * time.Second), value: i})
+	}
+	s.trimHistoryLocked(base.Add(time.Duration(maxHistoryPoints+25) * time.Second))
+	got := len(s.history)
+	s.mu.Unlock()
+
+	if got != maxHistoryPoints {
+		t.Fatalf("history length = %d, want %d", got, maxHistoryPoints)
+	}
+	if s.history[0].value != 25 {
+		t.Fatalf("oldest retained value = %d, want %d", s.history[0].value, 25)
+	}
+}