make extractOption less opinionated

- by returning falsy calcdata values
- use fillHoverText in choropleth/hover,
  for consistency

src/lib/index.js

@@ -447,8 +447,7 @@ lib.castOption = function(trace, ptNumber, astr, fn) {
  *  @return {any}
  */
 lib.extractOption = function(calcPt, trace, calcKey, traceKey) {
-    var calcVal = calcPt[calcKey];
-    if(calcVal || calcVal === 0) return calcVal;
+    if(calcKey in calcPt) return calcPt[calcKey];
 
     // fallback to trace value,
     //   must check if value isn't itself an array

src/traces/choropleth/hover.js

@@ -9,9 +9,9 @@
 
 'use strict';
 
-var Lib = require('../../lib');
 var Axes = require('../../plots/cartesian/axes');
 var attributes = require('./attributes');
+var fillHoverText = require('../scatter/fill_hover_text');
 
 module.exports = function hoverPoints(pointData, xval, yval) {
     var cd = pointData.cd;
@@ -81,8 +81,7 @@ function makeHoverInfo(pointData, trace, pt, axis) {
 
     if(hasZ) text.push(formatter(pt.z));
     if(hasText) {
-        var tx = Lib.extractOption(pt, trace, 'tx', 'text');
-        if(tx && tx !== '') text.push(tx);
+        fillHoverText(pt, trace, text);
     }
 
     pointData.extraText = text.join('<br>');

src/traces/scatter/fill_hover_text.js

@@ -29,8 +29,13 @@ module.exports = function fillHoverText(calcPt, trace, contOut) {
         function(v) { contOut.text = v; };
 
     var htx = Lib.extractOption(calcPt, trace, 'htx', 'hovertext');
-    if(htx && htx !== '') return fill(htx);
+    if(isValid(htx)) return fill(htx);
 
     var tx = Lib.extractOption(calcPt, trace, 'tx', 'text');
-    if(tx && tx !== '') return fill(tx);
+    if(isValid(tx)) return fill(tx);
 };
+
+// accept all truthy values and 0 (which gets cast to '0' in the hover labels)
+function isValid(v) {
+    return v || v === 0;
+}