function PathLine() {
    var d3Line = d3_svg_line().curve(d3_line_interpolate);

    function line(data) {
      return d3Line(data);
    }

    line.init = function(accessor_date, x, accessor_value, y, offset) {
      return d3Line.defined(function(d) { return accessor_value(d) !== null; })
          .x(function(d) { return x(accessor_date(d), offset === undefined ? offset : offset(d)); } )
          .y(function(d) { return y(accessor_value(d)); } );
    };

    line.d3 = function() {
      return d3Line;
    };

    return line;
  }
DataSelector.mapper.unity, DataSelector.mapper.array, or custom data mapper
@param _
@returns {*}