diff --git a/js/diagram.js b/js/diagram.js
index 75aa163..67e71a6 100644
--- a/js/diagram.js
+++ b/js/diagram.js
@@ -111,6 +111,45 @@ const Diagram = {
         //     case 'countryPopulation':
         //         break;
         // }
+
+    swap: (chords, i, j) => {
+        if (i < 0 || j < 0 || i === j) {
+            return chords;
+        }
+
+        // Determine which arc is closer to 0, so earlier arc always swaps with later.
+        const f = (chords.groups[i].endAngle < chords.groups[j].endAngle ? i : j);
+        const s = (chords.groups[i].endAngle < chords.groups[j].endAngle ? j : i);
+
+        const fst = Object.assign({}, chords.groups[f]);
+        const snd = Object.assign({}, chords.groups[s]);
+
+        const fstAngle = fst.endAngle - fst.startAngle;
+        const sndAngle = snd.endAngle - snd.startAngle;
+        const offsetAngle = sndAngle - fstAngle;
+
+        // Bring first forward.
+        chords.groups[f].startAngle = snd.endAngle - fstAngle;
+        chords.groups[f].endAngle = snd.endAngle;
+        // chords.groups[f].index = snd.index;
+
+        // Bring second back.
+        chords.groups[s].startAngle = fst.startAngle;
+        chords.groups[s].endAngle = fst.startAngle + sndAngle;
+        // chords.groups[s].index = fst.index;
+
+        // Bump other groups forward.
+        for (let ii = f + 1; ii < s; ii++) {
+            chords.groups[ii].startAngle += offsetAngle;
+            chords.groups[ii].endAngle += offsetAngle;
+        }
+
+        // Swap array positions.
+        const tmp = chords.groups[f]
+        chords.groups[f] = chords.groups[s];
+        chords.groups[s] = tmp;
+
+        return chords;
     },
 
     build: (data, eventIndex, metaMatrix, chordMatrix) => {