The Earned Value method is a little different from the classical approach for keeping track of project performance. It has an extra variable that represents the actual cost of the work done at a given point, and it is obtained from the organization 's accounting system. This data is compared with the earned value to show the amount of work that was planned with what was actually accomplished to determine if cost and schedule performance is as planned. So, it's possible, at any given point, to compare how much actual work has been completed against how much is expected to be completed and graph the results for quick referencing.
Example of Graph
Reading the above curve
CV is less than 0 : the cost of the actual work exceeded the value of the work done.
SV is greater than 0 : the quantity of work done is greater than the budgeted.
CPI is less than 1 : spending more than the value of the work done.
SPI is greater than 1 : more work done respect to the budgeted (ahead of schedule).
The above curve is just for illustration purposes and it was produced with Excel. It's possible to use a different curve and normalize all numbers so that the data will be uniformely distributed over the graph.