By default ColVis will use the information in the TH cell for each column as the button name to use in ColVis, which might not always be what you want (for example you might has HTML in the cell that you don't want in the button). The fnLabel callback provides the ability to customise the label used for the button. In this example the column index is prefixed to the column title.
$(document).ready( function () {
$('#example').dataTable( {
"sDom": 'C<"clear">lfrtip',
"oColVis": {
"fnLabel": function ( index, title, th ) {
return (index+1) +'. '+ title;
}
}
} );
} );