move initializer to constructor

src/components/continuous/histogramBrush.js

@@ -36,6 +36,7 @@ class HistogramBrush extends React.Component {
     this.width = 300;
     this.height = 100;
     this.marginBottom = 20;
+    this.histogramCache = {};
 
     this.state = {
       svg: null,
@@ -55,8 +56,6 @@ class HistogramBrush extends React.Component {
       nextProps.metadataField
     );
 
-    this.histogramCache = {};
-
     this.histogramCache.x = d3
       .scaleLinear()
       .domain([nextProps.ranges.min, nextProps.ranges.max])
@@ -81,7 +80,7 @@ class HistogramBrush extends React.Component {
   componentWillReceiveProps(nextProps) {
     if (
       this.props.metadataField !== nextProps.metadataField ||
-      !this.histogramCache
+      !this.histogramCache.x
     ) {
       this.calcHistogramCache(nextProps);
     }