Adds remove method to layer (#63).

examples/scripts/bar-chart.js

@@ -34,8 +34,7 @@ d3.chart("BarChart", {
 
     function onExitTrans() {
       this.duration(1000)
-          .attr("x", function(d, i) { return chart.x(i - 1) - .5; })
-          .remove();
+          .attr("x", function(d, i) { return chart.x(i - 1) - .5; });
     }
 
     function dataBind(data) {

src/layer.js

@@ -39,6 +39,15 @@ Layer.prototype.insert = function() {
 	d3cAssert(false, "Layers must specify an `insert` method.");
 };
 
+/**
+ * Invoked by {@link Layer#draw} in order to remove exiting DOM nodes from
+ * this layer's `base`. This default implementation may be overridden by
+ * Layer instances.
+ */
+Layer.prototype.remove = function() {
+	this.remove();
+};
+
 /**
  * Subscribe a handler to a "lifecycle event". These events (and only these
  * events) are triggered when {@link Layer#draw} is invoked--see that method
@@ -113,9 +122,12 @@ Layer.prototype.off = function(eventName, handler) {
 
 /**
  * Render the layer according to the input data: Bind the data to the layer
- * (according to {@link Layer#dataBind}, insert new elements (according to
- * {@link Layer#insert}, make lifecycle selections, and invoke all relevant
- * handlers (as attached via {@link Layer#on}) with the lifecycle selections.
+ * (according to {@link Layer#dataBind}), insert new elements (according to
+ * {@link Layer#insert}), make lifecycle selections, invoke all relevant
+ * handlers (as attached via {@link Layer#on}) with the lifecycle selections,
+ * then remove exiting elements (according to {@link Layer#remove}).
+ *
+ * The lifecycle selections are:
  *
  * - update
  * - update:transition
@@ -216,4 +228,6 @@ Layer.prototype.draw = function(data) {
 			}
 		}
 	}
+
+	this.remove.call(selection);
 };

test/tests/layer.js

@@ -82,6 +82,12 @@ suite("d3.layer", function() {
 			this.layer.draw([]);
 			assert(this.insert.calledOn(this.dataBind.returnValues[0].enter.returnValues[0]));
 		});
+		test("invokes the provided `remove` method", function() {
+			this.layer.draw([1]);
+			assert.equal(this.layer.selectAll('g')[0].length, 1);
+			this.layer.draw([]);
+			assert.equal(this.layer.selectAll('g')[0].length, 0);
+		});
 
 		suite("event triggering", function() {
 			test("invokes event handlers with the correct selection", function() {