Allow transient marks to be chained.

src/mark/Mark.js

@@ -1228,5 +1228,5 @@ pv.Mark.prototype.transition = function(ms) {
 };
 
 pv.Mark.prototype.on = function(state) {
-  return this["$" + state] = new pv.Transient();
+  return this["$" + state] = new pv.Transient(this);
 };

src/mark/Transient.js

@@ -1,6 +1,7 @@
-pv.Transient = function() {
+pv.Transient = function(mark) {
   pv.Mark.call(this);
   this.fillStyle(null).strokeStyle(null).textStyle(null);
+  this.on = function(state) { return mark.on(state); };
 };
 
 pv.Transient.prototype = pv.extend(pv.Mark);

tests/mark/transition-anchor.html

@@ -18,26 +18,21 @@
     .margin(30);
 
 var rule = vis.add(pv.Rule)
-    .data(function() i & 1 ? [0.25] : [0.5])
+    .data(function() i & 1 ? [0.25, 0.75] : [0.5])
     .id(function(d) d.toFixed(2))
     .bottom(function(d) d * h);
 
-var label = rule.anchor("left").add(pv.Label)
-    .text(function(d) d.toFixed(2));
-
-rule.enter()
-    .bottom(0)
-    .strokeStyle("transparent");
-
-rule.exit()
+rule.on("enter")
     .bottom(0)
-    .strokeStyle("transparent");
-
-label.enter()
-    .textStyle("transparent");
-
-label.exit()
-    .textStyle("transparent");
+    .left(0)
+    .width(0)
+  .on("exit")
+    .top(0)
+    .left(0)
+    .width(0);
+
+rule.anchor("left").add(pv.Label)
+    .text(function(d) d.toFixed(2));
 
 vis.render();