fix hexbin + dot sort (#891)

Author: mbostock

README.md

@@ -2250,8 +2250,6 @@ The following aggregation methods are supported:
 * a function to be passed the array of values for each bin and the extent of the bin
 * an object with a *reduce* method
 
-When the hexbin transform has an *r* output channel, bins are returned in order of descending radius.
-
 See also the [hexgrid](#hexgrid) mark.
 
 ### Custom initializers

src/channel.js

@@ -73,17 +73,20 @@ export function channelDomain(channels, facetChannels, data, options) {
   }
 }
 
-function sortInitializer(name, compare = ascendingDefined) {
+function sortInitializer(name, optional, compare = ascendingDefined) {
   return (data, facets, {[name]: V}) => {
-    if (!V) throw new Error(`missing channel: ${name}`);
+    if (!V) {
+      if (optional) return {}; // do nothing if given channel does not exist
+      throw new Error(`missing channel: ${name}`);
+    }
     V = V.value;
     const compareValue = (i, j) => compare(V[i], V[j]);
     return {facets: facets.map(I => I.slice().sort(compareValue))};
   };
 }
 
-export function channelSort(initializer, {channel, reverse}) {
-  return composeInitializer(initializer, sortInitializer(channel, reverse ? descendingDefined : ascendingDefined));
+export function channelSort(initializer, {channel, optional, reverse}) {
+  return composeInitializer(initializer, sortInitializer(channel, optional, reverse ? descendingDefined : ascendingDefined));
 }
 
 function findScaleChannel(channels, scale) {

src/marks/dot.js

@@ -28,7 +28,7 @@ export class Dot extends Mark {
         {name: "rotate", value: vrotate, optional: true},
         {name: "symbol", value: vsymbol, scale: "symbol", optional: true}
       ],
-      vr === undefined || options.sort !== undefined ? options : {...options, sort: {channel: "r", reverse: true}},
+      options.sort === undefined ? {...options, sort: {channel: "r", optional: true, reverse: true}} : options,
       defaults
     );
     this.r = cr;