A picture may be worth a thousand words.
The graph which appear below is priceless.      -Scott

Source: Natural Resource Economics by Barry C. Field, Professor of Resource Economics at the University of Massachusetts in Amherst
 
On the Use of Graphs in Economics 2:

Graphs may come in any number of different shapes, however. Suppose, for example, we are interested in the relationship between the amount of the entrance fee to visit a public park and the number of people who visit that park. We might expect a negative relationship, as depicted below:

Note that there are no actual numbers on the axes in this case. We are interested only in the general nature of the function, i.e., at higher entrance fees visitation will be lower. So the actual numbers on the scale need not be specified.

A point that we will come back to is the following: There are normally lots of factors that affect park visitation rates, not solely the entrance fee. But Figure 4 shows only the relationship between the two variables: entrance fee and visitation. We have to understand that this function expresses the connection between these two factors with other things assumed constant. In other words, at higher or lower entrance fees this function show how visitation will respond, on the assumption that all other factors that might affect visitation are remaining unchanged. Of course in the real world this is hardly ever true; everything is usually in constant change. The reason for singling out just these two variables and assuming everything else is constant is because we wish to explore the nature of the connection between these two important factors. Thus, in this particular graph (or model) we are temporally abstracting from other factors that might have an impact on visitation.

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