fix initializer composition (#896)

Author: mbostock

src/transforms/initializer.js

@@ -11,9 +11,9 @@ export function composeInitializer(i1, i2) {
   if (i1 == null) return i2 === null ? undefined : i2;
   if (i2 == null) return i1 === null ? undefined : i1;
   return function(data, facets, channels, scales, dimensions) {
-    let c1, c2;
-    ({data, facets, channels: c1} = i1.call(this, data, facets, channels, scales, dimensions));
-    ({data, facets, channels: c2} = i2.call(this, data, facets, {...channels, ...c1}, scales, dimensions));
-    return {data, facets, channels: {...c1, ...c2}};
+    let c1, d1, f1, c2, d2, f2;
+    ({data: d1 = data, facets: f1 = facets, channels: c1} = i1.call(this, data, facets, channels, scales, dimensions));
+    ({data: d2 = d1, facets: f2 = f1, channels: c2} = i2.call(this, d1, f1, {...channels, ...c1}, scales, dimensions));
+    return {data: d2, facets: f2, channels: {...c1, ...c2}};
   };
 }