hello guys, can i customize and make the line chart graph to have two lines because now i can only have one line, 

like example:
field name, amount_due, amount_paid

i make it like this:
<graph string="Sales Orders">
    <field name="name" type="col"/>
    <field name="amount_due" type="measure"/>
    <field name="amount_paid" type="measure"/>
</graph>​

Thanks in advance..