Most programs that used graphics on the
TI 99/4A created them in a character-based mode, somewhat like the
text mode graphics that DOS programs would use for years to come (for that matter, there are still DOS-mode programs in the latest versions of
Windows that use such graphics).
The biggest difference from other text modes is that characters on the TI were redefinable; you could tell the TI to use any 8x8 two-color bitmap as any character, then print that character on screen anywhere you needed your newly redefined character to appear. There were actually two sets of redefinable characters -- the first 32 characters of high ASCII stayed defined until you rebooted, while the 96 characters of regular ASCII, 32 to 127, could be redefined within a program but reverted to normal any time you returned to the BASIC interpreter prompt.
From BASIC, the call to redefine a character encoded the bitmap as a string of 16 hex digits, which represented the raw data, left to right and top to bottom, probably exactly as it was stored internally. I still have tons of those little graphics lying around as hex strings, though I haven't used a TI in over 10 years.
There was an expansion unit which, among other things, upgraded the display resolution and made character set be 16x16 bitmaps. I never had one, but sometimes in some of the computing magazines of the day, you'd see program listings that used 64-character hex strings to define those characters.