How to select only one column in a table using jQuery UI selectable?

What I am trying to do is allow the user to select rows from a single column at a time using jQuery UI selectable.

What I need to do though is restrict the drag/highlight effect from spanning more than one column. E.g. which ever column the user start the drag event in they cant highlight out side that column.

Table code

<table id="selectable">
  <tr>
    <td data-col="0">
    <td data-col="1">
    <td data-col="2">
  </tr>
  <tr>
    <td data-col="0">
    <td data-col="1">
    <td data-col="2">
  </tr>
  <tr>
    <td data-col="0">
    <td data-col="1">
    <td data-col="2">
  </tr>
  ...etc
</table>

jQuery:

var currentCol;

$("#selectable").selectable({
    filter:'td',
    selecting: function(event, ui){
        console.log($(ui.selected));
    }
});

But I cant access the value of data-col, it's always null.

Thanks to Daniel for pointing me in the right direction.

In order to lock the drag down top a single column, this is what I did.

    var currentCol;

    $("#selectable").selectable({
        filter: "td",
        start: function(event, ui) {
            $("td").removeClass("ui-selected");
        },
        stop: function(event, ui) {

            //Reset selector. 
            currentCol = undefined;
        },
        selecting: function(event, ui) {

            if (currentCol === undefined) {
                currentCol = $(ui.selecting).attr('data-col');
            }

            var nthChild = parseInt(currentCol) + 1; //Add one as nthChild is not zero index

            for (var i = 1; i <= 9; i++) {

                if (i != nthChild) {
                    $("td.ui-selecting:nth-child(" + i + ")").each(function() {
                        $(this).removeClass("ui-selecting");
                    });
                }
            }
            ;
        }
    });