Real numbers on a number line can (in theory) be plotted with as much precision as your number line.

If your number line has interval precision, then pi can be plotted somewhere between 3 and 4.

If your number line has n'th decimal place precision, then pi can be calculated to the n'th decimal and plotted.

An infinitely precise number line cannot exist in real life. Just how we can't calculate pi to infinite digits.