Vertical and horizontal charts could be stacked. Example:
<CHART width="750" height="400">
<SERIES>
<S type="vbar" x="lastname" y="sales" stack="syear"/>
</SERIES>
<SQL>
select first 80 * from (
select e.firstname, e.lastname, extract(year from saledate) syear,
sum(o.itemstotal) sales
from customer c
join orders o on o.custno=c.custno
join employee e on e.empno=o.empno
group by 1, 2,3
order by 1,2,3
)
</SQL>
</CHART>
Result:
