Other answers are irrelevant because they do not take into consideration this question is in a BIRT context. Short answer is this feature is not natively supported in BIRT. You can do this for labels in charts, but not for other elements such Data, Dynamic text etc.
You could include a css file at view time, and make use of a css rule in a text element declared as "HTML" type, but it is complicated to maintain and this would only work with html outputs: other formats such PDF and Excel would not take css rules into consideration.
Your best option is to make use of this Eclipse project BIRT control libs which extends BIRT in particular with a "diagonal text" control.