rank, quantile (#570)

* update dependencies

* rank and quantile map methods

* count, and fix off by one

* document rank and quantile map methods

Co-authored-by: Philippe Rivière <fil@rezo.net>

README.md

@@ -1319,6 +1319,8 @@ Like the [group](#group) and [bin](#bin) transforms, the [Plot.map](#plotmapoutp
 The following map methods are supported:
 
 * *cumsum* - a cumulative sum
+* *rank* - the rank of each value in the sorted array
+* *quantile* - the rank, normalized between 0 and 1
 * a function to be passed an array of values, returning new values
 * an object that implements the *map* method
 

src/transforms/map.js

@@ -1,4 +1,4 @@
-import {group} from "d3";
+import {count, group, rank} from "d3";
 import {maybeZ, take, valueof, maybeInput, lazyChannel} from "../mark.js";
 import {basic} from "./basic.js";
 
@@ -43,10 +43,17 @@ function maybeMap(map) {
   if (typeof map === "function") return mapFunction(map);
   switch (`${map}`.toLowerCase()) {
     case "cumsum": return mapCumsum;
+    case "rank": return mapFunction(rank);
+    case "quantile": return mapFunction(rankQuantile);
   }
   throw new Error("invalid map");
 }
 
+function rankQuantile(V) {
+  const n = count(V) - 1;
+  return rank(V).map(r => r / n);
+}
+
 function mapFunction(f) {
   return {
     map(I, S, T) {