Split three-stated variable into 2 variables.

src/scales/scale.category.js

@@ -61,17 +61,13 @@ module.exports = function() {
 			// This is the case in some tests, and probably only in tests
 			var ds = chart.getDatasetMeta && chart.getDatasetMeta(datasetIndex).controller;
 
-			// Three states here
-			// null:  There is no getValueScaleId in dataset (= not a bar chart)
-			// true:  Scale same as this
-			// false: Value scale is not this scale
-			var isValue = ds && ds.getValueScaleId
-				? ds.getValueScaleId() === me.id
-				: null;
-
-			// FIXME For non-bar charts we assume vertical axis is value scale
+			// Do we have a getValuesScaleId function? (eg. bar chart)
+			var hasValueScale = ds && ds.getValueScaleId;
+			var isValueScale = hasValueScale && ds.getValueScaleId() === me.id;
+
+			// FIXME For non-bar charts we assume vertical axis is a value scale
 			// and thus should return getRightValue
-			if (isValue || (isValue === null && !isHorizontal)) {
+			if (isValueScale || (!hasValueScale && !isHorizontal)) {
 				return me.getRightValue(data.datasets[datasetIndex].data[index]);
 			}
 			return me.ticks[index - me.minIndex];