Fix nearest interaction mode to return all items

Return all items that are at the nearest distance to the point.
Add unit tests for nearest + axis: 'x' and nearest + axis: 'y'

docs/general/interactions/modes.md

@@ -20,7 +20,7 @@ var chart = new Chart(ctx, {
 ```
 
 ## nearest
-Gets the item that is nearest to the point. The nearest item is determined based on the distance to the center of the chart item (point, bar). If 2 or more items are at the same distance, the one with the smallest area is used. If `intersect` is true, this is only triggered when the mouse position intersects an item in the graph. This is very useful for combo charts where points are hidden behind bars.
+Gets the items that are at the nearest distance to the point. The nearest item is determined based on the distance to the center of the chart item (point, bar). You can use the `axis` setting to define which directions are used in distance calculation. If `intersect` is true, this is only triggered when the mouse position intersects an item in the graph. This is very useful for combo charts where points are hidden behind bars.
 
 ```javascript
 var chart = new Chart(ctx, {

src/core/core.interaction.js

@@ -243,26 +243,7 @@ module.exports = {
 			var position = getRelativePosition(e, chart);
 			options.axis = options.axis || 'xy';
 			var distanceMetric = getDistanceMetricForAxis(options.axis);
-			var nearestItems = getNearestItems(chart, position, options.intersect, distanceMetric);
-
-			// We have multiple items at the same distance from the event. Now sort by smallest
-			if (nearestItems.length > 1) {
-				nearestItems.sort(function(a, b) {
-					var sizeA = a.getArea();
-					var sizeB = b.getArea();
-					var ret = sizeA - sizeB;
-
-					if (ret === 0) {
-						// if equal sort by dataset index
-						ret = a._datasetIndex - b._datasetIndex;
-					}
-
-					return ret;
-				});
-			}
-
-			// Return only 1 item
-			return nearestItems.slice(0, 1);
+			return getNearestItems(chart, position, options.intersect, distanceMetric);
 		},
 
 		/**