proposed filter api for d4

gistfile1.js

// create a chart
var columnChart = d4.baseChart()

// mix in a feature to the chart
.mixin([{name: 'bars', feature: d4.features.stacked-shapes-series}])

// when using the feature apply a filter to the rects.
.using('bars',function(bar){
  bar.svgFilter('feGaussianBlur', function(filter){
    filter
    .in("SourceGraphic")
    .stdDeviation("5");
  });
});