fix bug

lib/core.js

@@ -55,13 +55,16 @@ module.exports = {
       } else {
         // assign all of the properites from the next datasets to the current chart
         nextProps.data.datasets.forEach(function(set, setIndex) {
-          var chartDataset = chart.data.datasets[setIndex];
+
+          var chartDataset = {};
 
           for (var property in set) {
             if (set.hasOwnProperty(property)) {
               chartDataset[property] = set[property];
             }
           }
+
+          chart.data.datasets[setIndex] = chartDataset;
         });
 
         chart.data.labels = nextProps.data.labels;