function floatingTooltip(tooltipID, width) {
    /*
     * Creates tooltip with provided id that
     * floats on top of visualization.
     * Most styling is expected to come from CSS
     * so check out bubble_chart.css for more details.
     */
    // Local variable to hold tooltip div for
    // manipulation in other functions.
    var tt = d3.select('body')
        .append('div')
        .attr('class', 'tooltip')
        .attr('id', tooltipID)
        .style('pointer-events', 'none');

    // Set a width if it is provided.
    if (width) {
        tt.style('width', width);
    }

    // Initially it is hidden.
    hideTooltip();

    function showTooltip(content, event) {
        /*
         * Display tooltip with provided content.
         *
         * content is expected to be HTML string.
         *
         * event is d3.event for positioning.
         */
        tt.style('opacity', 1.0)
            .html(content);

        updatePosition(event);
    }

    function hideTooltip() {
        /*
         * Hide the tooltip div.
         */
        tt.style('opacity', 0.0);
    }

    function updatePosition(event) {
        /*
         * Figure out where to place the tooltip
         * based on d3 mouse event.
         */
        var xOffset = 20;
        var yOffset = 10;

        var ttw = tt.style('width');
        var tth = tt.style('height');

        var wscrY = window.scrollY;
        var wscrX = window.scrollX;

        var curX = (document.all) ? event.clientX + wscrX : event.pageX;
        var curY = (document.all) ? event.clientY + wscrY : event.pageY;
        var ttleft = ((curX - wscrX + xOffset * 2 + ttw) > window.innerWidth) ?
                                 curX - ttw - xOffset * 2 : curX + xOffset;

        if (ttleft < wscrX + xOffset) {
            ttleft = wscrX + xOffset;
        }

        var tttop = ((curY - wscrY + yOffset * 2 + tth) > window.innerHeight) ?
                                curY - tth - yOffset * 2 : curY + yOffset;

        if (tttop < wscrY + yOffset) {
            tttop = curY + yOffset;
        }

        tt.style('top', tttop + 'px');
        tt.style('left', ttleft + 'px');
    }

    return {
        showTooltip: showTooltip,
        hideTooltip: hideTooltip,
        updatePosition: updatePosition
    };
}