code: function addPointToDomain( point, x, y, domainId ){
  
          var domain = domains[ domainId ];
          var newPoint = {
              value: point,
              x: x,
              y: y,
              identifier: domain.identifier,
              domainId: domainId
          };
  
          pointsHash[ x + '_' + y ] = {
              value: point,
              identifier: domain.identifier,
              domainId: domainId
          };
  
          domain.points.push( newPoint );
      }
docs: add a point to a existing domain, and attach properties domainId and identifier to point.
  @param point
  @param x
  @param y
  @param domainId