Fix sankey.linkSort when non-null.

README.md

@@ -100,6 +100,10 @@ For convenience, a link’s source and target may be initialized using numeric o
 * *link*.width - the link’s width (proportional to *link*.value)
 * *link*.index - the zero-based index of *link* within the array of links
 
+<a name="sankey_linkSort" href="#sankey_linkSort">#</a> <i>sankey</i>.<b>linkSort</b>([<i>sort</i>]) [<>](https://github.com/d3/d3-sankey/blob/master/src/sankey.js "Source")
+
+If *sort* is specified, sets the link sort method and returns this Sankey generator. If *sort* is not specified, returns the current link sort method, which defaults to *undefined*, indicating that vertical order of links within each node will be determined automatically by the layout. If *sort* is null, the order is fixed by the input. Otherwise, the specified *sort* function determines the order; the function is passed two links, and must return a value less than 0 if the first link should be above the second, and a value greater than 0 if the second link should be above the first, or 0 if the order is not specified.
+
 <a name="sankey_nodeId" href="#sankey_nodeId">#</a> <i>sankey</i>.<b>nodeId</b>([<i>id</i>]) [<>](https://github.com/d3/d3-sankey/blob/master/src/sankey.js "Source")
 
 If *id* is specified, sets the node id accessor to the specified function and returns this Sankey generator. If *id* is not specified, returns the current node id accessor, which defaults to the numeric *node*.index:

src/sankey.js

@@ -138,6 +138,12 @@ export default function Sankey() {
       source.sourceLinks.push(link);
       target.targetLinks.push(link);
     }
+    if (linkSort != null) {
+      for (const {sourceLinks, targetLinks} of nodes) {
+        sourceLinks.sort(linkSort);
+        targetLinks.sort(linkSort);
+      }
+    }
   }
 
   function computeNodeValues({nodes}) {