code: function(ctx, baselineX, baselineY, pathBack) {
      ctx.lineTo(baselineX, baselineY);
      if (stackedGraph) {
        for (var i = pathBack.length - 1; i >= 0; i--) {
          var pt = pathBack[i];
          ctx.lineTo(pt[0], pt[1]);
        }
      }
    }
docs: for different line drawing modes (line/step) per series Helper function to trace a line back along the baseline.