Many numbers can be defined graphically. Some of the irrational ones can be found on the number line using quite simple graphs. Here are a few examples:
The square root of 2 ≈ 1.414 is the point at which the graph of y=x2-2 crosses the x axis.
Pi or 3.14 is the place where the sin() function first crosses the x-axis after leaving the origin.
e 2.71828 is often used with the exponential function ex. e is the value of y at x=1.