Serialize by expression source, not type.

Author: mbostock

lib/cube/metric.js

@@ -70,7 +70,7 @@ exports.getter = function(db) {
     var remaining = 0,
         time0 = Date.now(),
         time = start,
-        name = expression.type,
+        name = expression.source,
         queue = queueByName[name],
         step = tier.key;