It is possible to draw vertical lines using the DRAW commands.  Unfortunately, lines drawn with these commands cannot utilize any of the graphing features such as intersect, etc.

There is a way to "fake" the calculator into producing a somewhat "workable" vertical line. 

To graph the vertical line x = 5:

    Y1 = A Big Number (x - 5)

where "A Big Number" is around 1,000,000.

The intersection option can be engaged.