Slow subquery in SELECT statement

Having tried many different methods, I can't get my query taking shorter than 40 seconds to run with a dataset of ~11000 rows.

The table is a list of hotel bookings, containing (most relevant) Check-In date and Booking Value

My boss wanted a diagram where the values are summed for every month/year in a special way:

Row 1: Sum of value between Jan 2012 - Jan 2012 Row 2: Sum of value between Jan 2012 - Feb 2012 Row 3: Sum of value between Jan 2012 - Mar 2012 Row 4: Sum of value between Jan 2012 - Apr 2012 ... Row 5: Sum of value between Jan 2012 - Dec 2012

And so on for every year. Basically row 5 in this example would return the summed value for the entire year.

Table:

CREATE TABLE IF NOT EXISTS `bookings` (
  `DBID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `checkin` date NOT NULL,
  `checkout` date NOT NULL,
  `total` int(11) NOT NULL,
  PRIMARY KEY (`DBID`,`checkin`),
  KEY `total` (`total`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=11171 ;

My query:

SELECT
    YEAR(checkin) as year,
    CONCAT(UCASE(LEFT(MONTHNAME(checkin), 1)),SUBSTRING(MONTHNAME(checkin), 2)) as month,
    MONTH(checkin) as monthnum,
    (
        SELECT SUM(total)
        FROM bookings as book2
        WHERE YEAR(book2.checkin) = year AND MONTH(book2.checkin) <= monthnum
    ) as sum
FROM `bookings`
GROUP BY YEAR(checkin),MONTH(checkin)
ORDER BY YEAR(checkin) ASC, MONTH(checkin) ASC

So the subquery is the magic behind this query, it all produces the exact result I want, but the execution time is unacceptable.

I'm not sure how to optimise this further, I can't use joins since it's on the same table, I've indexed everything neccessary, etc.