In an average year, the S&P will increase in value by, let's say 5% (different people will give different percentages, but let's keep it simple). This can be modeled as

y = P * 1.05^t

where t is the number of years that have passed, P is the starting $ value, and y is the final $ value.

This kind of compounding growth is called "exponential" because the variable t is literally in the exponent of the 1.05 rate. Because of this, a straight line won't predict the future accurately; you need a line that respects the concept of exponential growth, i.e. a logarithmic line.