Fix bubble chart tooltip callback to use correct labels (#3421)

Fix bubble chart tooltip callback to use correct label parsed from scales. Fixes #3029

src/controllers/controller.bubble.js

@@ -31,7 +31,7 @@ module.exports = function(Chart) {
 				label: function(tooltipItem, data) {
 					var datasetLabel = data.datasets[tooltipItem.datasetIndex].label || '';
 					var dataPoint = data.datasets[tooltipItem.datasetIndex].data[tooltipItem.index];
-					return datasetLabel + ': (' + dataPoint.x + ', ' + dataPoint.y + ', ' + dataPoint.r + ')';
+					return datasetLabel + ': (' + tooltipItem.xLabel + ', ' + tooltipItem.yLabel + ', ' + dataPoint.r + ')';
 				}
 			}
 		}