What are the convection currents, and how do they affect the earth's asthenosphere

Alaina wants to decide whether her current cell phone plan is better than a package that another company offers. to compare the two plans, she plotted two lines on a graph, where y represented the total cost of one month’s service and x represented the megabytes of data that she used in a month. which part of the graph will represent the cost per megabyte of data used in each plan?