Normalize mixin API

Mixin declarations should be similar to te Layer and Chart APIs. Namely,
the first argument to `Chart#mixin` should be the Chart name, the second
should be the selection for the mixin's base, and all following
arguments should be passed through to the mixin Chart's constructor.

examples/scripts/hybrid.js

@@ -4,8 +4,8 @@ d3.chart("Hybrid", {
     var barHeight = this.barHeight();
     var barWidth = this.radius * 2;
 
-    var chord = this.chord = this.mixin(this.base.append("g"), "ImprovedChord");
-    var bc = this.bc = this.mixin(this.base.append("g"), "FadingBarChart");
+    var chord = this.chord = this.mixin("ImprovedChord", this.base.append("g"));
+    var bc = this.bc = this.mixin("FadingBarChart", this.base.append("g"));
     chord.transform = function(data) {
       return d3.chart("Chord").prototype.transform(data.series2);
     };

src/d3-chart.js

@@ -88,7 +88,7 @@
 		return data;
 	};
 
-	Chart.prototype.mixin = function(selection, chartName) {
+	Chart.prototype.mixin = function(chartName, selection) {
 		var args = Array.prototype.slice.call(arguments, 2);
 		args.unshift(selection);
 		var ctor = Chart[chartName];

test/tests/d3-chart.js

@@ -95,16 +95,16 @@ suite("d3.chart", function() {
 			this.myChart = d3.select("#test").chart("test");
 		});
 		test("instantiates the specified chart", function() {
-			var mixin = this.myChart.mixin(d3.select("body"), "test2", 1, 2, 45);
+			var mixin = this.myChart.mixin("test2", d3.select("body"), 1, 2, 45);
 			assert.instanceOf(mixin, d3.chart("test2"));
 		});
 		test("instantiates with the correct arguments", function() {
-			var mixin = this.myChart.mixin(d3.select("body"), "test2", 1, 2, 45);
+			var mixin = this.myChart.mixin("test2", d3.select("body"), 1, 2, 45);
 			assert.deepEqual(mixin.initialize.args[0], [1, 2, 45]);
 		});
 		test("correctly sets the `base` attribute of the mixin", function() {
 			var mixinBase = d3.select("body");
-			var mixin = this.myChart.mixin(mixinBase, "test2");
+			var mixin = this.myChart.mixin("test2", mixinBase);
 			assert.equal(mixin.base, mixinBase);
 		});
 	});
@@ -130,8 +130,8 @@ suite("d3.chart", function() {
 			});
 			sinon.spy(layer2, "draw");
 
-			this.mixin1 = mixin1 = myChart.mixin(d3.select("#test"), "test");
-			this.mixin2 = mixin2 = myChart.mixin(d3.select("#test"), "test");
+			this.mixin1 = mixin1 = myChart.mixin("test", d3.select("#test"));
+			this.mixin2 = mixin2 = myChart.mixin("test", d3.select("#test"));
 			sinon.stub(mixin1, "draw");
 			sinon.stub(mixin2, "draw");
 		});