explicit "auto" thresholds

Author: Fil

README.md

@@ -1039,6 +1039,7 @@ Plot.binX({y: "count"}, {x: {thresholds: 20, value: "culmen_length_mm"}})
 
 The **thresholds** option may be specified as a named method or a variety of other ways:
 
+* *auto* (default) - Scott’s rule, capped at 200.
 * *freedman-diaconis* - the [Freedman–Diaconis rule](https://en.wikipedia.org/wiki/Freedman–Diaconis_rule)
 * *scott* - [Scott’s normal reference rule](https://en.wikipedia.org/wiki/Histogram#Scott.27s_normal_reference_rule)
 * *sturges* - [Sturges’ formula](https://en.wikipedia.org/wiki/Histogram#Sturges.27_formula)
@@ -1047,7 +1048,7 @@ 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*, 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.
+If the **thresholds** option is specified as 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

@@ -177,6 +177,7 @@ function maybeThresholds(thresholds = thresholdDefault) {
       case "freedman-diaconis": return thresholdFreedmanDiaconis;
       case "scott": return thresholdScott;
       case "sturges": return thresholdSturges;
+      case "auto": return thresholdDefault;
     }
     throw new Error("invalid thresholds");
   }