A lot of proof-of-concept code renders to memory (and putpixel becomes screen[y*width+x]=pixel) and then saves the memory to a graphics file. If you've already been motivated to work with graphics you probably already know how to save a PNG file. If you have ImageMagick you can even render to a raw, uncompressed, non-standard graphics file and describe the format of the file in the 'display' command when you want to view the file. Also standard formats like BMP and TGA can be pretty easy to use without a library.