The graph unit is not available for Mac OS X because no one has implemented it. The source code that gets installed is generic and hence also contains the source code for units that don't work on the current platform (e.g., it also includes the source code for the Win32 rtl).
The sdlgraph unit was once started by someone, but that person unfortunately disappeared after a short while. You can try using it by added "sldgraph" to your uses clause, but it's unlikely that it will work very well. Contributions to make it work better are of course welcome.