cap the default number of bins at 200 (see #417)

Author: Fil

README.md

@@ -1031,7 +1031,7 @@ The **thresholds** option may be specified as a named method or a variety of oth
 * a time interval (for temporal binning)
 * a function that returns an array, count, or time interval
 
-If the **thresholds** option is not specified, it defaults to *scott*. If a function, it is passed three arguments: the array of input values, the domain minimum, and the domain maximum. If a number, [d3.ticks](https://github.com/d3/d3-array/blob/master/README.md#ticks) or [d3.utcTicks](https://github.com/d3/d3-time/blob/master/README.md#ticks) is used to choose suitable nice thresholds.
+If the **thresholds** option is not specified, it defaults to *scott*, capped at 200. If a function, it is passed three arguments: the array of input values, the domain minimum, and the domain maximum. If a number, [d3.ticks](https://github.com/d3/d3-array/blob/master/README.md#ticks) or [d3.utcTicks](https://github.com/d3/d3-time/blob/master/README.md#ticks) is used to choose suitable nice thresholds.
 
 The bin transform supports grouping in addition to binning: you can subdivide bins by up to two additional ordinal or categorical dimensions (not including faceting). If any of **z**, **fill**, or **stroke** is a channel, the first of these channels will be used to subdivide bins. Similarly, Plot.binX will group on **y** if **y** is not an output channel, and Plot.binY will group on **x** if **x** is not an output channel. For example, for a stacked histogram:
 

src/transforms/bin.js

@@ -171,7 +171,10 @@ function maybeBin(options) {
   return bin;
 }
 
-function maybeThresholds(thresholds = thresholdScott) {
+function maybeThresholds(thresholds) {
+  if (thresholds === undefined) {
+    return (...args) => Math.min(200, thresholdScott(...args));
+  }
   if (typeof thresholds === "string") {
     switch (thresholds.toLowerCase()) {
       case "freedman-diaconis": return thresholdFreedmanDiaconis;