Thanks Kaz, another small update today. With any luck perhaps I'll be able to get this released this year. ->link<- (not sure how to embed the video, could you please edit this for me?)
They are quite straight-forward and well written for the time as more or less an unrolled version of Bresenham's to handle the various angle groupings. But the C64 source has the option to use 'draw_fastline.asm' rather than 'draw_line.asm'.
As the DrawLine function is basically a start x,y and an end x,y then routine could be a drop-in substitution. Here, as the narrow screen (32 bytes wide) layout is employed, that could be taken into consideration.
But plenty of other stuff to munch through to get this near completion so not a priority.
(Note I'm not using the online BBC or C64 sources but those based on my own disassembly I did around the turn of the century - I've used the online ones this year to cross-reference and help debug the issues I was having)