Monopoly's MC, AVC and TVC I do not quite understand the statement "total variable cost is the area under the marginal cost curve" Does it means MC inserts at the AVC at the lowest point (see picture)? But why AVC is drawn and not TVC?
Total variable cost would not fit on that graph: total cost has units of "dollars", while the axes of the graph are "items" and "dollars per item"

But you can multiply the two together, which implies that areas on the graph are measured in dollars. 

For example, below the pale blue area is the total variable cost for the quantity shown, while the pink area is the total cost