//function to visualize functions[0] = function(x) { return sin(x*PI); } //function coloring options functionColor[0] = "red"; functionLineWidth[0] = 2; //background rendering options backgroundColor = "black"; gridColor = "#202020";

You can draw custom graphics on the canvas by setting up a draw function.
draw = function() { c2d.fillStyle = "white"; c2d.font = "20px sans-serif"; c2d.fillText("Example Label",40,40); };
The draw function is called after everything else. Find full documentation of Javascript canvas drawing functions here.