The slope of the graph of a line is constant. If the line is y=mx, m represents the slope and is the ratio of y to x. This means that every point on the line (that is, every (x,y) value), gives two numbers which are in the same constant ratio to one another. For example, y=⅔x means that y/x=2/3 or y:x=2:3. Take the point (9,6), 6:9=2:3 and the point (12,8). 8:12=2:3. So the graph shows the equivalent ratio.