diff --git a/app/routes/api.py b/app/routes/api.py
index f8e83c3..cb74eeb 100644
--- a/app/routes/api.py
+++ b/app/routes/api.py
@@ -1,11 +1,120 @@
-from flask import Blueprint, jsonify, session
+from flask import Blueprint, jsonify, request, session
 from app import SYDNEY_TZ
 from app.auth import login_required
 from app.db import query, query_one
-from datetime import timezone
+from datetime import datetime, timezone
 
 bp = Blueprint("api", __name__, url_prefix="/api")
 
+# Distinct hues for up to 12 users; cycles if more
+_CHART_HUES = [210, 160, 270, 30, 340, 190, 50, 120, 300, 80, 240, 10]
+
+
+@bp.route("/progress-over-time")
+@login_required
+def progress_over_time():
+    """Return per-user % weight lost over time for the leaderboard line chart."""
+    # --- parse optional filters ---
+    start = request.args.get("start")  # ISO date string
+    end = request.args.get("end")
+    user_ids_raw = request.args.get("user_ids", "")
+
+    # Build WHERE fragments
+    where_clauses = ["u.is_private = FALSE"]
+    params = []
+
+    if start:
+        where_clauses.append("c.checked_in_at >= %s")
+        params.append(start)
+    if end:
+        where_clauses.append("c.checked_in_at < (%s::date + interval '1 day')")
+        params.append(end)
+    if user_ids_raw:
+        try:
+            uid_list = [int(x) for x in user_ids_raw.split(",") if x.strip()]
+        except ValueError:
+            uid_list = []
+        if uid_list:
+            placeholders = ",".join(["%s"] * len(uid_list))
+            where_clauses.append(f"u.id IN ({placeholders})")
+            params.extend(uid_list)
+
+    where_sql = " AND ".join(where_clauses)
+
+    rows = query(f"""
+        SELECT
+            u.id                    AS user_id,
+            u.display_name,
+            u.username,
+            u.starting_weight_kg,
+            (SELECT weight_kg FROM checkins
+             WHERE user_id = u.id ORDER BY checked_in_at ASC LIMIT 1) AS first_weight,
+            c.weight_kg,
+            c.checked_in_at
+        FROM checkins c
+        JOIN users u ON u.id = c.user_id
+        WHERE {where_sql}
+        ORDER BY u.id, c.checked_in_at ASC
+    """, params)
+
+    # Group rows by user
+    from collections import OrderedDict
+    users_map = OrderedDict()
+    for r in rows:
+        uid = r["user_id"]
+        if uid not in users_map:
+            start_w = float(r["starting_weight_kg"] or r["first_weight"] or 0)
+            users_map[uid] = {
+                "id": uid,
+                "name": r["display_name"] or r["username"],
+                "start_w": start_w,
+                "data": [],
+            }
+        entry = users_map[uid]
+        w = float(r["weight_kg"])
+        dt = r["checked_in_at"]
+        if dt.tzinfo is None:
+            dt = dt.replace(tzinfo=timezone.utc)
+        entry["data"].append({
+            "date": dt.astimezone(SYDNEY_TZ).strftime("%Y-%m-%d"),
+            "weight": round(w, 1),
+        })
+
+    # Build response with best-fit
+    result = []
+    for idx, (uid, entry) in enumerate(users_map.items()):
+        hue = _CHART_HUES[idx % len(_CHART_HUES)]
+        color = f"hsl({hue}, 70%, 55%)"
+
+        # Simple linear regression (x = day index, y = weight)
+        points = entry["data"]
+        best_fit = {"slope": 0, "intercept": 0}
+        if len(points) >= 2:
+            # Convert dates to day offsets from first point
+            base = datetime.strptime(points[0]["date"], "%Y-%m-%d")
+            xs = [(datetime.strptime(p["date"], "%Y-%m-%d") - base).days for p in points]
+            ys = [p["weight"] for p in points]
+            n = len(xs)
+            sum_x = sum(xs)
+            sum_y = sum(ys)
+            sum_xy = sum(x * y for x, y in zip(xs, ys))
+            sum_x2 = sum(x * x for x in xs)
+            denom = n * sum_x2 - sum_x * sum_x
+            if denom != 0:
+                slope = (n * sum_xy - sum_x * sum_y) / denom
+                intercept = (sum_y - slope * sum_x) / n
+                best_fit = {"slope": round(slope, 4), "intercept": round(intercept, 4)}
+
+        result.append({
+            "id": uid,
+            "name": entry["name"],
+            "color": color,
+            "data": points,
+            "best_fit": best_fit,
+        })
+
+    return jsonify({"users": result})
+
 
 @bp.route("/chart-data/<int:user_id>")
 @login_required
diff --git a/app/static/css/style.css b/app/static/css/style.css
index 9a73c20..08394b9 100644
--- a/app/static/css/style.css
+++ b/app/static/css/style.css
@@ -814,6 +814,94 @@ tr:hover td {
     box-shadow: 0 0 0 3px var(--accent-glow);
 }
 
+/* ========== CHART FILTERS ========== */
+.chart-filters {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 1rem;
+    align-items: flex-start;
+    padding: 0 0 1rem;
+}
+
+.filter-group {
+    display: flex;
+    flex-direction: column;
+    gap: 0.35rem;
+}
+
+.filter-group label {
+    font-size: 0.75rem;
+    font-weight: 500;
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+    color: var(--text-muted);
+}
+
+.filter-group input[type="date"] {
+    width: 160px;
+    padding: 0.5rem 0.75rem;
+    background: var(--bg-input);
+    border: 1px solid var(--border);
+    border-radius: var(--radius-sm);
+    color: var(--text-primary);
+    font-family: inherit;
+    font-size: 0.85rem;
+    outline: none;
+    transition: all 0.2s ease;
+}
+
+.filter-group input[type="date"]:focus {
+    border-color: var(--border-focus);
+    box-shadow: 0 0 0 3px var(--accent-glow);
+}
+
+.filter-group-people {
+    flex: 1;
+    min-width: 200px;
+}
+
+.person-filter-list {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 0.5rem;
+    max-height: 72px;
+    overflow-y: auto;
+    padding: 0.25rem 0;
+}
+
+.person-checkbox {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.35rem;
+    padding: 0.3rem 0.65rem;
+    background: var(--bg-secondary);
+    border: 1px solid var(--border);
+    border-radius: 100px;
+    font-size: 0.8rem;
+    color: var(--text-secondary);
+    cursor: pointer;
+    transition: all 0.2s ease;
+    user-select: none;
+}
+
+.person-checkbox:hover {
+    border-color: var(--accent);
+    color: var(--text-primary);
+}
+
+.person-checkbox input[type="checkbox"] {
+    accent-color: var(--accent);
+    width: 14px;
+    height: 14px;
+    cursor: pointer;
+}
+
+.person-checkbox:has(input:checked) {
+    background: var(--accent-glow);
+    border-color: rgba(59, 130, 246, 0.3);
+    color: var(--accent);
+}
+
 /* ========== RESPONSIVE ========== */
 @media (max-width: 768px) {
     .navbar {
diff --git a/app/static/js/charts.js b/app/static/js/charts.js
index 777a1ed..83f1d59 100644
--- a/app/static/js/charts.js
+++ b/app/static/js/charts.js
@@ -144,3 +144,113 @@ function createComparisonChart(canvasId, names, pctLost) {
         },
     });
 }
+
+
+/**
+ * Create a multi-series progress-over-time line chart with best-fit lines.
+ * @param {string} canvasId - canvas element ID
+ * @param {Array} users - array of { id, name, color, data: [{date, pct_lost}], best_fit: {slope, intercept} }
+ * @returns {Chart} the Chart.js instance
+ */
+function createProgressChart(canvasId, users) {
+    const ctx = document.getElementById(canvasId);
+    if (!ctx) return null;
+
+    const datasets = [];
+
+    users.forEach(user => {
+        // Actual data line
+        datasets.push({
+            label: user.name,
+            data: user.data.map(d => ({ x: d.date, y: d.weight })),
+            borderColor: user.color,
+            backgroundColor: user.color.replace(')', ', 0.1)').replace('hsl(', 'hsla('),
+            fill: false,
+            tension: 0.3,
+            pointBackgroundColor: user.color,
+            pointBorderColor: '#1a2233',
+            pointBorderWidth: 2,
+            pointRadius: 4,
+            pointHoverRadius: 6,
+            borderWidth: 2.5,
+        });
+
+        // Best-fit line (dashed) — only if we have ≥ 2 data points
+        if (user.data.length >= 2) {
+            const baseDate = new Date(user.data[0].date);
+            const firstDay = 0;
+            const lastDay = Math.round(
+                (new Date(user.data[user.data.length - 1].date) - baseDate) / 86400000
+            );
+
+            const fitStart = user.best_fit.intercept + user.best_fit.slope * firstDay;
+            const fitEnd = user.best_fit.intercept + user.best_fit.slope * lastDay;
+
+            datasets.push({
+                label: user.name + ' (trend)',
+                data: [
+                    { x: user.data[0].date, y: Math.round(fitStart * 100) / 100 },
+                    { x: user.data[user.data.length - 1].date, y: Math.round(fitEnd * 100) / 100 },
+                ],
+                borderColor: user.color,
+                borderDash: [6, 4],
+                borderWidth: 2,
+                pointRadius: 0,
+                pointHoverRadius: 0,
+                fill: false,
+                tension: 0,
+            });
+        }
+    });
+
+    return new Chart(ctx, {
+        type: 'line',
+        data: { datasets },
+        options: {
+            ...chartDefaults,
+            scales: {
+                x: {
+                    type: 'time',
+                    time: {
+                        unit: 'day',
+                        tooltipFormat: 'dd MMM yyyy',
+                        displayFormats: { day: 'dd MMM' },
+                    },
+                    grid: { color: 'rgba(42, 53, 72, 0.5)', drawBorder: false },
+                    ticks: { color: '#64748b', font: { size: 11, family: 'Inter' } },
+                },
+                y: {
+                    ...chartDefaults.scales.y,
+                    title: {
+                        display: true,
+                        text: 'Weight (kg)',
+                        color: '#64748b',
+                        font: { size: 12, family: 'Inter' },
+                    },
+                },
+            },
+            plugins: {
+                ...chartDefaults.plugins,
+                legend: {
+                    display: true,
+                    labels: {
+                        color: '#94a3b8',
+                        font: { size: 12, family: 'Inter' },
+                        usePointStyle: true,
+                        pointStyle: 'circle',
+                        padding: 16,
+                        // Hide trend lines from legend
+                        filter: item => !item.text.endsWith('(trend)'),
+                    },
+                },
+                tooltip: {
+                    ...chartDefaults.plugins.tooltip,
+                    callbacks: {
+                        label: ctx => `${ctx.dataset.label}: ${ctx.parsed.y} kg`,
+                    },
+                },
+            },
+        },
+    });
+}
+
diff --git a/app/templates/base.html b/app/templates/base.html
index 50ab092..2b34050 100644
--- a/app/templates/base.html
+++ b/app/templates/base.html
@@ -1,5 +1,6 @@
 <!DOCTYPE html>
 <html lang="en">
+
 <head>
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
@@ -7,11 +8,15 @@
     <meta name="description" content="Track your weight loss competition with friends">
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800&display=swap" rel="stylesheet">
+    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700;800&display=swap"
+        rel="stylesheet">
     <link rel="stylesheet" href="{{ url_for('static', filename='css/style.css') }}">
     <script src="https://unpkg.com/htmx.org@2.0.4"></script>
     <script src="https://cdn.jsdelivr.net/npm/chart.js@4.4.7/dist/chart.umd.min.js"></script>
+    <script
+        src="https://cdn.jsdelivr.net/npm/chartjs-adapter-date-fns@3.0.0/dist/chartjs-adapter-date-fns.bundle.min.js"></script>
 </head>
+
 <body>
     {% if session.get('user_id') %}
     <nav class="navbar">
@@ -26,15 +31,18 @@
                 <span class="nav-link-icon">📊</span>
                 <span>Dashboard</span>
             </a>
-            <a href="{{ url_for('checkin.index') }}" class="{{ 'active' if request.endpoint and request.endpoint.startswith('checkin') }}">
+            <a href="{{ url_for('checkin.index') }}"
+                class="{{ 'active' if request.endpoint and request.endpoint.startswith('checkin') }}">
                 <span class="nav-link-icon">✏️</span>
                 <span>Check-in</span>
             </a>
-            <a href="{{ url_for('leaderboard.index') }}" class="{{ 'active' if request.endpoint == 'leaderboard.index' }}">
+            <a href="{{ url_for('leaderboard.index') }}"
+                class="{{ 'active' if request.endpoint == 'leaderboard.index' }}">
                 <span class="nav-link-icon">🏆</span>
                 <span>Leaderboard</span>
             </a>
-            <a href="{{ url_for('profile.index') }}" class="{{ 'active' if request.endpoint and request.endpoint.startswith('profile') }}">
+            <a href="{{ url_for('profile.index') }}"
+                class="{{ 'active' if request.endpoint and request.endpoint.startswith('profile') }}">
                 <span class="nav-link-icon">👤</span>
                 <span>Profile</span>
             </a>
@@ -67,4 +75,5 @@
 
     {% block scripts %}{% endblock %}
 </body>
-</html>
+
+</html>
\ No newline at end of file
diff --git a/app/templates/leaderboard.html b/app/templates/leaderboard.html
index 1bb4098..b7f8ab3 100644
--- a/app/templates/leaderboard.html
+++ b/app/templates/leaderboard.html
@@ -7,13 +7,37 @@
     <p>Ranked by % body weight lost. May the best loser win!</p>
 </div>
 
-<!-- Comparison Chart -->
+<!-- Progress-Over-Time Chart -->
 <div class="card" style="margin-bottom: 1.5rem;">
     <div class="card-header">
-        <h2>📊 % Weight Lost Comparison</h2>
+        <h2>📈 Progress Over Time</h2>
     </div>
-    <div class="chart-container">
-        <canvas id="comparisonChart"></canvas>
+
+    <!-- Filters -->
+    <div class="chart-filters">
+        <div class="filter-group">
+            <label for="filterStart">From</label>
+            <input type="date" id="filterStart" class="form-input">
+        </div>
+        <div class="filter-group">
+            <label for="filterEnd">To</label>
+            <input type="date" id="filterEnd" class="form-input">
+        </div>
+        <div class="filter-group filter-group-people">
+            <label>People</label>
+            <div class="person-filter-list" id="personFilterList">
+                {% for u in ranked %}
+                <label class="person-checkbox">
+                    <input type="checkbox" value="{{ u.id }}" checked>
+                    <span>{{ u.display_name or u.username }}</span>
+                </label>
+                {% endfor %}
+            </div>
+        </div>
+    </div>
+
+    <div class="chart-container" style="position: relative; height: 380px;">
+        <canvas id="progressChart"></canvas>
     </div>
 </div>
 
@@ -90,14 +114,50 @@
 {% block scripts %}
 <script src="{{ url_for('static', filename='js/charts.js') }}"></script>
 <script>
-    document.addEventListener('DOMContentLoaded', function () {
-        fetch('/api/comparison')
+    let _progressChart = null;
+    let _debounceTimer = null;
+
+    function loadProgressChart() {
+        const start = document.getElementById('filterStart').value;
+        const end = document.getElementById('filterEnd').value;
+
+        // Gather checked user IDs
+        const checks = document.querySelectorAll('#personFilterList input[type=checkbox]:checked');
+        const userIds = Array.from(checks).map(cb => cb.value).join(',');
+
+        const params = new URLSearchParams();
+        if (start) params.set('start', start);
+        if (end) params.set('end', end);
+        if (userIds) params.set('user_ids', userIds);
+
+        fetch('/api/progress-over-time?' + params.toString())
             .then(r => r.json())
             .then(data => {
-                if (data.names.length > 0) {
-                    createComparisonChart('comparisonChart', data.names, data.pct_lost);
+                if (_progressChart) {
+                    _progressChart.destroy();
+                    _progressChart = null;
+                }
+                if (data.users && data.users.length > 0) {
+                    _progressChart = createProgressChart('progressChart', data.users);
                 }
             });
+    }
+
+    function debouncedLoad() {
+        clearTimeout(_debounceTimer);
+        _debounceTimer = setTimeout(loadProgressChart, 300);
+    }
+
+    document.addEventListener('DOMContentLoaded', function () {
+        // Initial load
+        loadProgressChart();
+
+        // Wire up filter changes
+        document.getElementById('filterStart').addEventListener('change', debouncedLoad);
+        document.getElementById('filterEnd').addEventListener('change', debouncedLoad);
+        document.querySelectorAll('#personFilterList input[type=checkbox]').forEach(cb => {
+            cb.addEventListener('change', loadProgressChart);
+        });
     });
 </script>
 {% endblock %}
\ No newline at end of file