Dynamically populated select using ajax in WordPress - ajax

Question

I want to use the value from my first dropdown in a MySQL query to generate a second dropdown. I'm doing this in wordpress and have tried to modify code I have used for a form plugin to apply to a coded form. But the code I have doesn't populate the second dropdown.
<form id="page-changer" action="" method="post"> <?php
$chart_types = $wpdb->get_results( "SELECT page_id, title FROM master_chart WHERE mod_id=$mod_id AND (geo_type=$geo_type OR geo_type IS NULL) ORDER BY sequence" );
echo '<select id="chart_type" required style="width: 100%; margin-bottom: 15px;"><option value="" disabled selected>Select Chart Type</option>';
foreach ($chart_types as $chart_type) :
echo '<option value="'.$chart_type->page_id.'">'.$chart_type->title.'</option>';
endforeach;
echo '</select>'; ?>
<script type="text/javascript">
jQuery(document).ready(function(){
jQuery('#chart_type').change(function(){
var chartPOP=jQuery('#chart_type').val();
jQuery('#response').empty();
jQuery.ajax({
url:"<?php bloginfo( 'wpurl' ); ?>/wp-admin/admin-ajax.php",
type:'POST',
data:'action=populate_chart&pageID=' + chartPOP,
success:function(results) {
jQuery('#response').append(results);
}
});
});
});
</script> <?php
echo '<select id="response"><option value="" disabled selected>Select Frequency</option></select>';
function populate_chart() {
if( isset( $_POST['pageID'] ) ) :
$page_id=$_POST['pageID'];
global $wpdb;
$frequencies = $wpdb->get_results( "SELECT title FROM master_chartmeta WHERE measure >= $measure_toggle AND page_id=$page_id");
foreach( $results as $rows ) :
$option .= '<option value="'.$rows->title.'">';
$option .= $rows->title;
$option .= '</option>';
endforeach;
echo $option;
die();
endif;
}
add_action( 'wp_ajax_nopriv_populate_chart', populate_chart );
add_action( 'wp_ajax_populate_chart', populate_chart );