code: function(){
      if (this.seriesData) return this.seriesData;
  
      var s = this.series,
          rows = {};
  
      /* The data grid is a 2 dimensions array. There is a row for each X value.
       * Each row contains the x value and the corresponding y value for each serie ('undefined' if there isn't one)
      **/
      _.each(s, function(serie, i){
        _.each(serie.data, function (v) {
          var x = v[0],
              y = v[1],
              r = rows[x];
          if (r) {
            r[i+1] = y;
          } else {
            var newRow = [];
            newRow[0] = x;
            newRow[i+1] = y;
            rows[x] = newRow;
          }
        });
      });
  
      // The data grid is sorted by x value
      this.seriesData = _.sortBy(rows, function(row, x){
        return parseInt(x, 10);
      });
      return this.seriesData;
    }
docs: Builds a matrix of the data to make the correspondance between the x values and the y values :
  X value => Y values from the axes
  @return {Array} The data grid