Add a Distance/Time toggle to the profile chart's x-axis

TrackStats.build_profile now also computes elapsed_secs per point
(relative to the first known timestamp), alongside the existing
cumulative distance. Point x-positions stay distance-based either
way — only the shared axis strip's 5 tick labels (and the tooltip)
switch between the two, so toggling is a cheap re-render rather than
rebuilding the charts. The toggle only appears when a track actually
has elapsed-time data to switch to.

public/js/track_profile.js

@@ -55,9 +55,23 @@ export class TrackProfilePanel {
     this._tooltip.hidden = true;
     this._chartsContainer.appendChild(this._tooltip);
 
+    // Only offer the toggle when there's actually elapsed-time data to
+    // switch to — plenty of GPX files have no <time> at all.
+    this._hasTime = this._profile.some((p) => typeof p.elapsed_secs === "number");
+    this._xAxisMode = "distance";
+    if (this._hasTime) this._chartsContainer.appendChild(this._renderXAxisToggle());
+
     this._activeSeries = SERIES.filter((s) => this._profile.some((p) => typeof p[s.key] === "number"));
     this._activeSeries.forEach((series) => this._chartsContainer.appendChild(this._renderChart(series)));
 
+    // One shared strip below the whole stack, not per-chart — every chart
+    // above uses the same x-scale (distance-position), so a separate axis
+    // per chart would just repeat itself.
+    this._xAxisStripEl = document.createElement("div");
+    this._xAxisStripEl.className = "profile-panel__xaxis";
+    this._chartsContainer.appendChild(this._xAxisStripEl);
+    this._renderXAxisTicks();
+
     this._chartsContainer.addEventListener("pointermove", this._onPointerMove);
     this._chartsContainer.addEventListener("pointerleave", this._onPointerLeave);
 
@@ -108,6 +122,37 @@ export class TrackProfilePanel {
     return wrapper;
   }
 
+  _renderXAxisToggle() {
+    const row = document.createElement("div");
+    row.className = "profile-panel__xaxis-toggle";
+    row.innerHTML = `
+      <button type="button" class="profile-panel__xaxis-toggle-btn" data-mode="distance">Distance</button>
+      <button type="button" class="profile-panel__xaxis-toggle-btn" data-mode="time">Time</button>
+    `;
+    row.querySelectorAll("button").forEach((btn) => {
+      btn.classList.toggle("active", btn.dataset.mode === this._xAxisMode);
+      btn.addEventListener("click", () => {
+        if (btn.dataset.mode === this._xAxisMode) return;
+        this._xAxisMode = btn.dataset.mode;
+        row.querySelectorAll("button").forEach((b) => b.classList.toggle("active", b === btn));
+        this._renderXAxisTicks();
+      });
+    });
+    return row;
+  }
+
+  _renderXAxisTicks() {
+    const TICK_COUNT = 5;
+    this._xAxisStripEl.innerHTML = Array.from({ length: TICK_COUNT }, (_, i) => {
+      const fraction = i / (TICK_COUNT - 1);
+      const point = this._profile[nearestIndex(this._profile, fraction * this._maxDistance)];
+      const label = this._xAxisMode === "time" && typeof point.elapsed_secs === "number"
+        ? formatDuration(point.elapsed_secs)
+        : `${(point.distance_meters / 1000).toFixed(1)} km`;
+      return `<span class="profile-panel__xaxis-tick" style="left:${(fraction * 100).toFixed(2)}%">${escapeHtml(label)}</span>`;
+    }).join("");
+  }
+
   // Bound once (arrow fields, not prototype methods) so add/removeEventListener see the same reference.
   _onPointerMove = (event) => {
     const rect = this._chartsContainer.getBoundingClientRect();
@@ -158,7 +203,8 @@ export class TrackProfilePanel {
       .filter((s) => typeof point[s.key] === "number")
       .map((s) => `<div><span class="profile-panel__tooltip-label">${escapeHtml(s.label)}</span> <strong>${formatValue(point[s.key], s)}</strong></div>`)
       .join("");
-    this._tooltip.innerHTML = `<div class="profile-panel__tooltip-distance">${(point.distance_meters / 1000).toFixed(2)} km</div>${rows}`;
+    const timeSuffix = typeof point.elapsed_secs === "number" ? ` · ${formatDuration(point.elapsed_secs)}` : "";
+    this._tooltip.innerHTML = `<div class="profile-panel__tooltip-distance">${(point.distance_meters / 1000).toFixed(2)} km${timeSuffix}</div>${rows}`;
     this._tooltip.hidden = false;
 
     const containerRect = this._chartsContainer.getBoundingClientRect();