Sales report by month and item category

Hello

I'm currently using following query to report the monthly revenue of an online shop:
(well…I just use pgsql on a KISS basis ;o)

select date_trunc('month', orders.orderdate) as month, sum(orderitems.price * orderitems.orderitems2quantity) as revenue
from orders, orderitems, product
where orderitems.orderitems2orderid = orders.orderid
and orderitems.orderitems2productid = product.productid
group by date_trunc('month', orders.orderdate)
order by month desc;

gives:

month | revenue

------------------------+----------

2015-08-01 00:00:00+02 | 1956.00

2015-07-01 00:00:00+02 | 13079.40

2015-06-01 00:00:00+02 | 10864.20

But as the "product" items have categories assigned to them I would like to be able to also
report based on categories, something like:

month | bikes | cars | planes | submarines

------------------------+----------+----------+----------+------------

2015-08-01 00:00:00+02 | 233.00 | 4211.00 | 7833.50 | 723.35

So far I'm using two queries called in a loop in my PHP code….but the problem there is that
not all categories return a revenue for a specific month if nothing was sold in that month.

Or is that not possible in a more or less simple query?

thanks in advance
richard