Lets suppose your function is f(t)= t²+3t-6 then ,let Y axis be f(t) and X axis be just t where t is obviously time . Now put t=0,1,2,3, . And plot it you will get a parabola and now find f'(t), which is derivative of f(t) and your will get 2t+3 which is a linear equations and hence gives a straight line and your can plot this straight line on the graph which will tell you how the function is changing with respect to time .