I could have said d=distance we will see later in calculus d is reserved for something else.we will say y=distance and in this axis we will say time and i can say t=time but rather say x=time x=time, so if we had to plot Usain Bolt's distance as a function of time well in time 0 he hasn't gone anywhere he's right over there and we know at this gentleman is capable of travelling a 100 metres in 9.58 seconds so after 9.58 seconds we'll assume that this is in seconds over here, he is capable of going 100 metres.