Use TAB to switch focus from the plot and the input box. Type the function in the text box and hit ENTER to update. To change the "plot" parameters, press TAB to switch focus away from the text box. (The textbox has a blue outline if in focus.)
Commands:
CTRL+Left\Right or mouse drag: manual sweep
SHIFT+Up\Down: change wait between repetitions
SHIFT+Left\Right: change duration of each sweep
SHIFT+<\>: change frequency multiplier. Default is 4, or 2 octaves per vertical unit.
SHIFT+[-]\[+]: change frequency at y=0.
All typical math functions are available, and also sgn(x) and step(x). Use "x*x" or pow(x,2) for squaring.
There's also sqwave(x,n), which is an additive synthesis of a square wave with n terms.