add percentages to pieChart example

workaround for #703

not enough data is currently available to write a `.label()`
function that displays percentages, so this demonstrates using a
pretransition hook to correct the labels

web/examples/pie.html

@@ -28,7 +28,13 @@
     .innerRadius(100)
     .dimension(runDimension)
     .group(speedSumGroup)
-    .legend(dc.legend());
+    .legend(dc.legend())
+    // workaround for #703: not enough data is accessible through .label() to display percentages
+    .on('pretransition', function(chart) {
+        chart.selectAll('text.pie-slice').text(function(d) {
+            return d.data.key + ' ' + dc.utils.printSingleValue((d.endAngle - d.startAngle) / (2*Math.PI) * 100) + '%';
+        })
+    });
 
   chart.render();
 });