To graph a function, you just need to enter it into the Enter Function text box. 

Enter a function

Currently, you must enter the function in plain type. There are some prebuilt buttons if you want to use special characters or functions. You can graph multiple functions by separating them with a comma.

Multiple functions to graph

You can also populate the function box from a graph you have already entered in your Doc. Select the graph in your Doc and click on the Edit from Doc button. 

Edit From Doc with Zoom GIF