jQuery(document).ready(function($) {
    var hashArray = {};
    $('.myCheckbox').on('change', function() {
        var self = $(this),
            checkboxId = parseInt(self.attr('data-id')), // since you used it as a number above
            checkboxValue = parseInt(self.val()); // since you also used it as a number above
        if (self.is(':checked')) {
            // add to array
            if (!hashArray.hasOwnProperty(checkboxId)) hashArray[checkboxId] = [];
            hashArray[checkboxId].push(checkboxValue);
        }
        else {
            if (hashArray.hasOwnProperty(checkboxId)) {
                // remove from array if it exists
                hashArray[checkboxId].splice(hashArray[checkboxId].indexOf(checkboxValue), 1);
            }
        }
    });
});