You needed to loop all the objects in this
(function($){
$.fn.underline = function(options) {
var defaults = {
width: 1,
distance: 0,
color: '#000',
durationOn: 250,
durationOff: 250,
extend: 2,
};
var options = $.extend(defaults, options);
for(i = 0; i < this.length; i++)
{
var row = this[i];
var line = $("<div class=\"underlineLine\"></div>");
var position = $(row).offset();
var top = position.top;
var left = position.left;
var objWidth = $(row).width();
var objHeight = $(row).height();
$(line).css({'position' :'absolute',
'display' :'none',
'height' : options.width+'px',
'background-color': options.color,});
$(line).css({'left' : left-options.extend,
'top' : top+objHeight+options.distance,
'width': objWidth+options.extend*2 }).fadeIn();
$('body').append(line);
}
};
})(jQuery);