You can use either javascript code or normal URLs as a link.
Points that have an onclick event (they have a link set) will turn the mouse to a hand pointer
and will pulse to indecate something will happen if you click on them.
for( $i=0; $i<12; $i++ ) { $tmp = rand(14,19); // <-- we need to remember this value for the onClick $line_1->add_link( $tmp, 'javascript:alert("Downloads, '. $x_labels[$i] .' = '. $tmp .'")' );