Retrieve data from mysql by php to create flot graph

You probably want something like this. I haven't used flot but I looked at the example here.

<?php
//create array of pairs of x and y values
$dataset1 = array();
while ($row = mysql_fetch_assoc()) { //or whatever
    $dataset1[] = array( $row['xvalue'], $row['yvalue'] );
}
?>

<script type="text/javascript">
    //put array into javascript variable
    var dataset1 = <?php echo json_encode($dataset1); ?>;

    //plot
    $(function () {
         $.plot($("#placeholder"), [ dataset1 ]);
    });
</script>

This depends largely on your environment and requirements. There's lots of free tools out there you can use. One example is Flot that lets you use jQuery to build graphs. There's link to documentation on the Google Code page.