[PD] rotating coordinate system

[Achim Bornhoeft]

Hello,
I need to solve the following problem for different setups of room  
distributions.

Given are x and y values between 0 and 100:

0/100        100/100
|  	
|   x
|
|	x		
|____________
0/0          100/0

I would like to transform the same values mathematically in a way that  
the result correspond to the coordinate system rotated 45 degrees  
clockwise:

0/0---------0/100
|  	
|   	x
|
|   x		
|
100/0       100/100

Hopefully someone can help me with this.
Many thanks in advance.
Achim