Graphic commands are available only on computers with a graphics card (all PC's that run Windows have a graphics card).
It is nice to be able to make simple graphs, but the full topic is beyond the scope of this introduction. Program LIMTHS.BAS draws lines and boxes of various colors, in the display rectangle covering a portion of the screen.
If you are seriously interested in graphing the results of your computations, and printing the outcomes, you may want to switch to Visual Basic in Windows.