General functional notation (e.g. f(x) ) is not supported. If you wish to draw a function such as f(x) = 1 - |x|, replace f(x) with y (or any other unused variable).

So, to graph "f(x) = 1 - |x|", just type "y = 1 - |x|", click on "Solve" and you will get the following graph: