anchor.diff:  
     svg.append("g")
         .attr("class", "y axis")
  -      .call(yAxis)
  +      .call(yAxisWeight)
       .append("text")
         .attr("transform", "rotate(-90)")
         .attr("y", 6)
  
positive.code:   stroke-width: 1.5px;
  }
  
  </style>
  <body>
  <script src="http://d3js.org/d3.v3.js"></script>
  <script>
  
  var margin = {top: 20, right: 20, bottom: 30, left: 50},
      width = 960 - margin.left - margin.right,
      height = 500 - margin.top - margin.bottom;
  
  var parseDate = d3.time.format("%Y-%m-%d").parse;
  
negative.code:   });
  
    x.domain(d3.extent(data, function(d) { return d.measurementDate; }));
    yWeight.domain(d3.extent(data, function(d) { return d.weight; }));
    yBf.domain(d3.extent(data, function(d) { return d.bodyFat; }));
  
  
    svg.append("g")
        .attr("class", "x axis")
  