You can organize your expression list using Folders. To add a folder to your graph, start by opening the Add Item menu.
To add an expression into a folder, click and hold the icon next to the expression and drag the icon into the folder. You'll know that it's inside the folder if a gray line appears next to the expression line.
You can click the arrow next to the folder name to expand/collapse the folder contents and clicking the folder icon will show/hide the contents.