categorical vs continuous mini histograms (#827)

* comment

* add histogram functionality to Dataframe; port category occupancy to use it

* fix binning and create histogram for continous by catagorical

* Remove unnecessary logs

* Begin work on KDE

* Replace broken KDE with working histogram

* Define domain and range based on data from histogram

* Fix occupancy

* Add continuous obs and switch to canvas

* Stop value from always rerendering

* clear before render

* Clear canvas on render

* refactor categorical occupancy to canvas

* Remove log

* simplify finding max

* refactor kde->histogram and occupancy->bins

* refactor svg -> canvas

* rename to occupancy stack

* create popup

* add metadata and categorical values to popup

* fix overflow

* remove zeros info

* style graph

* fix shouldComponentUpdate to look for world changes

* change categorySelected -> categoryValueSelected

* refactor out render

* remove comment

* conditionally have bottom border

* remove diff comp

* remove comments

* remove unnecessary mapping

* Add comments describing drawing functions

* comments

* flip comparison order

* remove logging

* move default to parameter

* move defaults to parameter

* disable popover if not showing histogram

* fix wording and styling

* add line break

client/src/util/dataframe/util.js

@@ -5,6 +5,10 @@ Private utility code for dataframe
 export { isTypedArray, isArrayOrTypedArray } from "../typeHelpers";
 
 export function callOnceLazy(f) {
+  /*
+  call function once, and save the result, regardless of arguments (this is not
+  the same as typical memoization).
+  */
   let value;
   let calledOnce = false;
   const result = function result(...args) {
@@ -14,6 +18,25 @@ export function callOnceLazy(f) {
     }
     return value;
   };
-
   return result;
 }
+
+export function memoize(fn, hashFn) {
+  /* 
+  function memoization, with user-provided hash.  hashFn must return a
+  key which will be unique as a Map key (ie, obeys "sameValueZero" algorithm
+  as defined in the JS spec).  For more info on hash key, see:
+  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#Key_equality
+  */
+  const cache = new Map();
+  const wrap = function wrap(...args) {
+    const key = hashFn(...args);
+    if (cache.has(key)) {
+      return cache.get(key);
+    }
+    const result = fn(...args);
+    cache.set(key, result);
+    return result;
+  };
+  return wrap;
+}