code: function buildScales(){
              xScale = d3Scale.scaleTime()
                  .domain(d3Array.extent(data, getDate ))
                  .range([0, chartWidth]);
  
              yScale = d3Scale.scaleLinear()
                  .domain([0, d3Array.max(data, getValue)])
                  .range([chartHeight, 0]);
          }
docs: Creates the x and y scales of the graph
  @private