>Does Ruby have any modules useful in graphing equations like y=x**2+5,
>y**2+x**2=16, 4y+3x=28, and the like?

gnuplot does this: http://gnuplot.info/
Gnuplot can create your graphs as files or display them on the screen.
Gnuplot is ordinarily used as a standalone program which reads a
script, but you can make it work with your program by putting gnuplot
at the end of a pipeline. Your program writes gnuplot commands to
standard-out:

ruby myprog.rb | gnuplot

If you need to do something more complicated (or if your OS doesn't
support pipes), creating an object that acts as an output stream to a
gnuplot process would be fairly easy.