history-counters: Ensure last counter value can be displayed in tooltip

Previously, it was hard to display the most recent counter value in
histogram tooltip. This is no longer the case with this change.

swh/web/webapp/assets/webapp/history-counters.js

 /**
- * Copyright (C) 2019  The Software Heritage developers
+ * Copyright (C) 2019-2024  The Software Heritage developers
  * See the AUTHORS file at the top-level directory of this distribution
  * License: GNU Affero General Public License version 3, or any later version
  * See top-level LICENSE file for more information
@@ -136,8 +136,10 @@ export async function drawHistoryCounterGraph(container, historyData) {
 
   function updateTooltip(event) {
     const x0 = xScale.invert(d3.pointer(event)[0]);
-    const i = bisectDate(historyData, x0, 1);
-    if (i >= historyData.length) return;
+    let i = bisectDate(historyData, x0, 1);
+    if (i >= historyData.length) {
+      i = historyData.length - 1;
+    }
     const d0 = historyData[i - 1];
     const d1 = historyData[i];
     const d = x0 - d0[0] > d1[0] - x0 ? d1 : d0;