Allow float64 to down-cast to float32 (#472)

* warn if annotations will down-cast to float32

* correctly handle falsey data values

client/src/components/brushableHistogram/index.js

@@ -34,7 +34,7 @@ class HistogramBrush extends React.Component {
       .scaleLinear()
       .range([this.height - this.marginBottom, 0]);
 
-    if (obsAnnotations[0][field]) {
+    if (obsAnnotations[0][field] !== undefined) {
       // recalculate expensive stuff
       const allValuesForContinuousFieldAsArray = _.map(obsAnnotations, field);