VISITORS TBL
ID  product_id  hits    date
130     1       1   2012-08-11
94     810      53  2012-08-06
95     810      32  2012-08-07
96     810      22  2012-08-09
97     810      13  2012-08-10
98     810      16  2012-08-11
99     810      13  2012-08-22

<?
$rs = mysql_query("SELECT hits,date FROM visitors WHERE product_id=81 AND date BETWEEN 2012-08-01 AND 2012-08-31 ORDER BY date ASC");
$flot = array();
$last_date = '2012-08-00';
$day = 1;

if ($rs && mysql_num_rows($rs)) {
    while ($row = mysql_fetch_array($rs)) {
        $days_since = check_days_since($row['date'], $last_date);
        print $row['date'] . '|' . $last_date . '|' . $days_since . "n";

        if ($days_since) {
            for ($i = 1; $i < $days_since; $i++) {
                $flot[] = array($day++, 0);
            }
        }

        $flot[] = array($day++, $row['hits']);
        $last_date = $row['date'];
    }
}

print json_encode($flot);


function check_days_since($new, $old) {
    if (!$new || !$old) return 1;
    return (strtotime($new) - strtotime($old)) / (60*60*24);
}
?>