refactoring

docs/HistogramView.js

@@ -3,13 +3,15 @@ class HistogramView {
     constructor() {
     }
 
-    show(batchcode, div) {
+    show(batchcode, uiContainer) {
         fetch(`data/histograms/${batchcode}.json`)
             .then(response => response.json())
             .then(json => {
                 const data = json.histograms[3].histogram;
+                const canvas = document.createElement("canvas");
+                uiContainer.appendChild(canvas);
                 new Chart(
-                    div.querySelector('#symptomByBatchcodeHisto'),
+                    canvas,
                     {
                         type: 'bar',
                         data: {