for different line drawing modes (line/step) per series Helper function to trace a line back along the baseline.
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]);
      }
    }
  }