Assembler, Pascal and C were popular languages. Graphics were generated by directly interfacing with the display hardware, for instance the Color Graphics Adapter. (CGA)
Probably there were libraries, but not like today, and libraries were often not shared outside a company.
A resource which would have been relevant in say, 1988, can be found here:
https://web.archive.org/web/20170910230837/http://gd.tuwien.ac.at/languages/c/programming-bbrown/advcw1.htm