Vertify ticks is an array and update docs

docs/axes/README.md

@@ -31,7 +31,7 @@ There are a number of config callbacks that can be used to change parameters in
 | `beforeDataLimits` | `axis` | Callback that runs before data limits are determined.
 | `afterDataLimits` | `axis` | Callback that runs after data limits are determined.
 | `beforeBuildTicks` | `axis` | Callback that runs before ticks are created.
-| `afterBuildTicks` | `axis`, `ticks` | Callback that runs after ticks are created. Useful for filtering ticks (*Should return the filtered ticks*).
+| `afterBuildTicks` | `axis`, `ticks` | Callback that runs after ticks are created. Useful for filtering ticks. Should return the filtered ticks.
 | `beforeTickToLabelConversion` | `axis` | Callback that runs before ticks are converted into strings.
 | `afterTickToLabelConversion` | `axis` | Callback that runs after ticks are converted into strings.
 | `beforeCalculateTickRotation` | `axis` | Callback that runs before tick rotation is determined.

src/core/core.scale.js

@@ -291,7 +291,7 @@ module.exports = Element.extend({
 	afterBuildTicks: function(ticks) {
 		var me = this;
 		// ticks is empty for old axis implementations here
-		if (ticks.length) {
+		if (helpers.isArray(ticks) && ticks.length) {
 			return helpers.callback(me.options.afterBuildTicks, [me, ticks]);
 		}
 		// Support old implementatios (that modified `this.ticks` directly in buildTicks)