When looking at graphs, such as the marginal benefits and costs of different levels of pollution reduction along the riverfront, how do we know which economic principles, benefits, and costs were considered when making the chart and which still have to be considered?
This is a very curious thought that Evan cooked up. It is true that we don't necessarily know the exact economic principles that were considered in the chart. However, since the chart does in fact show the difference in marginal benefit, cost of control, and other factors, it is a good chart to use in this case. We were trying to see the difference at the margin in class and that is what the chart gave us. When it comes to economics, there are many factors that are often unseen along with the chart. That is why all of the famous economists (including Professor Weaver) talk about "Secondary Effects" all the time. Therefore, I agree with Evan that an economic chart has limitations, but a chart can't explain every single principle that economics has to offer.
The chart was simply an example, but if for the sake of an academic conversation, if the chart says "total" for the costs and benefits, than we can safely assume ALL possible costs and benefits have been included in the numbers given at each percentage.