First up is some code to clip 16-bit line coordinates down to 8-bit coordinates. This allows for lines to be partially (or completely) off the screen.
He's also written a fast ellipse drawing and filling routine. The ellipses are also clipped to the viewport and are filled with an 8x8 pixel pattern.
The graphics viewport can be redefined using the VDU 24,left;top;right;bottom; command as demonstrated in the above example.
GCOL can also be used to set a plotting mode; either plotting the specified colour directly, performing a logical operation (OR, AND, EOR) or inverting the existing colour.
All but the last of the above screenshots are the result of running BBC BASIC on a TI-83+ SE at 15MHz. The final screenshot is running at the regular 6MHz.