var factory = d3.svg.area()
      .x( function(d) { return graph.x(d.x) } )
      .y0( function(d) { return graph.y(d.y0) } )
      .y1( function(d) { return graph.y(d.y + d.y0) } )
      .interpolate(this.graph.interpolation).tension(this.tension);