If you need to rename the labels to something that does not exist on the database then the instructions below show how axis labels can be overridden.
If an Optional Y Series Grouping is added to a Report then whatever is displayed on the X-Axis is automatically defaulted in as the Sort Order. This means that if the X-Axis is displaying Month Names then they will be sorted alphabetically, which is incorrect.
http://www.birt-exchange.org/org/forum/index.php/topic/ 10988-changing-x-axis-labels-only/