I wrote a function graphing tool that might be useful for others to use. Sometimes I use graphtoy.com in my work but needed something a little more customized.
Type in your own expressions and graph the output.
Written in BlitzmaxNG and uses OpenGL.

Mouse Wheel and Up/Down key -> zoom graph
mouse left click and drag -> pan graph
‘r’ -> reset graph window
‘enter’ -> update editing function
‘escape’ -> quit

Notes from update: 0.2
   Added saving png image of graph  (ctrl-p)
   Functions can now reference other functions (beware of recursion) EX: “f1(x)+f2(x)”
   enable copy and paste with clipboard (ctrl-c, ctrl-v)
   smoothed line drawing
   added popup of built in functions (key F1)
   fixes some bugs.

Update: 0.3
Fixed a few syntax errors
Added sliders named A,B,C,D, expressions can reference these values.

Download exe and source-code