Question 311155
It's parallel to the y-axis, which is often called vertical.
It's parallel because for any value of y, x = 4.  It goes thru the points
(4,0), (4,4), (4,100), any value of y.  That makes it 4 units to the right of the y-axis, parallel to it --> "vertical"
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In 3 axis systems, the y-axis is sometimes horizontal and the z-axis if "vertical."  It's an arbitrary assignment.  If you graph x = 4 and it's vertical, then rotate your graph paper 90º, then it's horizontal.  Use caution in calling it vertical.