fix bug in histogram min/max aggregation with normalization

Author: alexcjohnson

src/traces/histogram/bin_functions.js

@@ -47,8 +47,9 @@ module.exports = {
                 return v;
             }
             else if(size[n] > v) {
+                var delta = v - size[n];
                 size[n] = v;
-                return v - size[n];
+                return delta;
             }
         }
         return 0;
@@ -63,8 +64,9 @@ module.exports = {
                 return v;
             }
             else if(size[n] < v) {
+                var delta = v - size[n];
                 size[n] = v;
-                return v - size[n];
+                return delta;
             }
         }
         return 0;