Treat the larger as the source.

README.md

@@ -49,7 +49,7 @@ Each source and target subgroup is also an object with the following properties:
 * `value` - the flow value *matrix*[*i*][*j*]
 * `index` - the node index *i*
 
-The chords are typically passed to [d3.ribbon](#ribbon) to display the network relationships. The returned array includes only chord objects for which the value *matrix*[*i*][*j*] or *matrix*[*j*][*i*] is non-zero. Furthermore, the returned array only contains unique chords: a given chord *ij* represents the bidirectional flow from *i* to *j* *and* from *j* to *i*, and does not contain a duplicate chord *ji*.
+The chords are typically passed to [d3.ribbon](#ribbon) to display the network relationships. The returned array includes only chord objects for which the value *matrix*[*i*][*j*] or *matrix*[*j*][*i*] is non-zero. Furthermore, the returned array only contains unique chords: a given chord *ij* represents the bidirectional flow from *i* to *j* *and* from *j* to *i*, and does not contain a duplicate chord *ji*; *i* and *j* are chosen such that the chord’s source always represents the larger of *matrix*[*i*][*j*] and *matrix*[*j*][*i*].
 
 The *chords* array also defines a secondary array of length *n*, *chords*.groups, where each group represents the combined outflow for node *i*, corresponding to the elements *matrix*[*i*][0 … *n* - 1], and is an object with the following properties:
 

src/chord.js

@@ -75,14 +75,20 @@ function chord(directed, transpose) {
           const subgroupIndex = range(0, n).filter(j => matrix[i * n + j] || matrix[j * n + i]);
           if (sortSubgroups) subgroupIndex.sort((a, b) => sortSubgroups(matrix[i * n + a], matrix[i * n + b]));
           for (const j of subgroupIndex) {
+            let chord;
             if (i < j) {
-              const chord = chords[i * n + j] || (chords[i * n + j] = {source: null, target: null});
+              chord = chords[i * n + j] || (chords[i * n + j] = {source: null, target: null});
               chord.source = {index: i, startAngle: x, endAngle: x += matrix[i * n + j] * k, value: matrix[i * n + j]};
             } else {
-              const chord = chords[j * n + i] || (chords[j * n + i] = {source: null, target: null});
+              chord = chords[j * n + i] || (chords[j * n + i] = {source: null, target: null});
               chord.target = {index: i, startAngle: x, endAngle: x += matrix[i * n + j] * k, value: matrix[i * n + j]};
               if (i === j) chord.source = chord.target;
             }
+            if (chord.source && chord.target && chord.source.value < chord.target.value) {
+              const source = chord.source;
+              chord.source = chord.target;
+              chord.target = source;
+            }
           }
           groups[i] = {index: i, startAngle: x0, endAngle: x, value: groupSums[i]};
         }