Make sure Calculator isn't running, and then copy the "Graphing-2D.calcview" folder from Calculator.app -> Contents -> Resources (control-click and select "Show Package Contents" to get the Contents folder) to Calculator.app -> Contents -> Plugins. Now start the Calculator app. There will now be a menu item named "Graphing" in the View menu of the Calculator app. This displays the (rather primitive) Graphing window.
Although not extremely useful, it does help in desperate situations (as a student, I know these occur rather often!). BTW, this is not the only .calcview plugin you can activate -- use the same trick to activate the Expression Sheet and Hexadecimal views.

