Question 307165
The x-axis is horizontal.
The y-axis is vertical.
If you connect two points that have the same x value, (4,0) and (4,8) for instance. 
The line will be parallel to the y-axis because the x value does not change. Since the lines are parallel, then the line x=4 is also vertical. 
{{{ drawing( 300, 300, -2, 8, -10, 10, grid(1),green(line(4,-10,4,10)),circle(4,8,.2),circle(4,0,.2),graph(300,300,-2,8,-10,10))) }}}