Some linear functions are known as constant functions. They are characterized by vertical or horizontal lines that run parallel to the y or x axes. These functions assume one of two forms. In the first form, the value of y remains the same regardless of the position you plot relative to the x axis. This line runs parallel to the x axis and is distinguished only by the changing value of x.
y = a
In y = a, a identifies a constant value. When you plot such a function, the graph assumes the form shown in Figure 10.8. You equate the lines this figure shows as a flat slope. In other words, the slope has no rise, only a run. For all values of x, y remains the same.