Added functionality to clear the chart canvas so charts can be more easily created

assets/scripts/reports.coffee

@@ -1,4 +1,6 @@
 $ ->
+  chartInstance = null
+
   $('#reports-download').on 'click', ->
     canvas = $('#benchmark-chart')[0]
     a = document.createElement 'a'
@@ -7,6 +9,8 @@ $ ->
     a.click()
 
   $('#reports-button').on 'click', (e) ->
+    chartInstance.destroy() if chartInstance
+
     benchmarkId = $('#report-benchmarks').val()
     testIds = $('#report-tests').val()
 
@@ -67,7 +71,7 @@ $ ->
         y:
           beginAtZero: true
 
-    new Chart ctx,
+    chartInstance = new Chart ctx,
       type: 'bar'
       data: data
       options: options