jQuery AJAX
Google Visualization API
Google Visualization API: Intensity Map
ISO 3166 Country Codes (1)
ISO 3166 Country Codes (2)

<script src="http://www.google.com/jsapi"></script>
<script>
    // Load jQuery
    google.load("jquery", "1.2.6", {uncompressed:false});

    // Load Google Visualization API
    google.load('visualization', '1', {packages: ['table', 'intensitymap']});
</script>
<script>
    function drawIntensityMap() {
        $.ajax({
            type: "POST",
            url: "wwv_flow.show",
            data: {p_flow_id:$('#pFlowId').attr("value"), 
                   p_flow_step_id:$('#pFlowStepId').attr("value"), 
                   p_instance:$('#pInstance').attr("value"), 
                   p_request:"APPLICATION_PROCESS=GET_VISITORS_PER_COUNTRY"
            },
            dataType: "json",
            success: function(data, textStatus){
                if ((nb_country = data.length) > 0) {
                    var map_data = new google.visualization.DataTable();                    
                    map_data.addColumn('string', 'Country');
                    map_data.addColumn('number', 'Visitors');
                    map_data.addRows(nb_country);
                    
                    for (i = 0; i < nb_country; i++) {
                        map_data.setCell(i, 0, data[i].country);
                        map_data.setCell(i, 1, Number(data[i].visitors));
                    }
                    
                    var imap = new google.visualization.IntensityMap($x('intensitymap_div'));
                    imap.draw(map_data, {});
                }
            }
        });
    }

    $().ready(function() {
        drawIntensityMap();
    });
</script>

DECLARE
    l_retval    VARCHAR2(32767);
    l_rows      VARCHAR2(32767);
BEGIN
    FOR i IN (SELECT cc.code
                    ,cv.visitors      
              FROM   jq_country_visitors cv
                    ,jq_iso_3166_country_codes cc
              WHERE  cv.country = cc.country)
    LOOP
        l_rows := l_rows||'{''country'':'''||i.code||''',''visitors'':'''||i.visitors||'''},';
    END LOOP;
    
    l_retval := '['||rtrim(l_rows,',')||']';
    
    htp.prn(l_retval);
END;