I think the answer to your question is in the C code. Apparently, the memory of the LCD is split into a graphics (bit addressable) area and a text are (to store character codes from the internal CG ROM or user defined characters in RAM). Look for the #defines for G_BASE and T_BASE. These area's visually 'overlay'; using the MODE SET you can choose to logically OR, AND or XOR these layers so that you can combine text and graphics. Not much different from most other LCD controllers as it seems. If you have sufficient room in your controller's data memory you can also opt to define your own characters and display them in graphics mode ...