How to replace an area chart with a line chart?

Question

I'm very new to d3js and i'm trying to modify the example : https://blockbuilder.org/lorenzopub/9d030cdfe0f3564fd5d7b9047e440084. I would like to replace the first area with a simple line plot. I changed only these lines of code:

//old: generators.local.area = d3.area(localTaxData).x(function(d){return generators.shared.x(d.year);})
//   .y0(function(d){return generators.federal.y(0);}).y1(function(d){return generators.federal.y(d.taxRate);})
//   .curve(d3.curveMonotoneX);

generators.local.area = d3.line(localTaxData).x(function(d){return generators.shared.x(d.year);})
  .y(function(d){return generators.federal.y(d.taxRate);}).curve(d3.curveMonotoneX);

but I get: can some one help me?