add the *mode* aggregation

the plot example shows the most common word of length x as a title for each bar

closes #420

README.md

@@ -978,6 +978,7 @@ The following aggregation methods are supported:
 * *median* - the median value
 * *deviation* - the standard deviation
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
+* *mode* - the value with the most occurrences
 * a function to be passed the array of values for each bin
 * an object with a *reduce* method
 

src/transforms/group.js

@@ -1,4 +1,4 @@
-import {group as grouper, sort, sum, deviation, min, max, mean, median, variance} from "d3";
+import {group as grouper, sort, sum, deviation, min, max, mean, median, mode, variance} from "d3";
 import {firstof} from "../defined.js";
 import {valueof, maybeColor, maybeInput, maybeTransform, maybeTuple, maybeLazyChannel, lazyChannel, first, identity, take, labelof, range} from "../mark.js";
 
@@ -148,6 +148,7 @@ export function maybeReduce(reduce, value) {
     case "mean": return reduceAccessor(mean);
     case "median": return reduceAccessor(median);
     case "variance": return reduceAccessor(variance);
+    case "mode": return reduceAccessor(mode);
   }
   throw new Error("invalid reduce");
 }

test/plots/word-length-moby-dick.js

@@ -21,7 +21,7 @@ export default async function() {
       percent: true
     },
     marks: [
-      Plot.barY(words, Plot.groupX({y: "proportion"}, {x: "length"}))
+      Plot.barY(words, Plot.groupX({y: "proportion", title: "mode"}, {x: "length", title: d => d}))
     ]
   });
 }