Linear equations are usually represented in it's slope-intercept form. This form is given by:
On this case "m" is the slope of the line and "b" is the y-intercept of the equation. On this case "m" is the speed of the cat, while "b" is the head-start. If we change the head-start we are changing the point at which the function crosses the y-axis.
When changing the speed of the graph the inclination of the graph changes, this happens because the slope of the graph is the speed.