Determining limiting nutrients from a graph
The control graph shows the baseline against which we can compare the other graphs. It is the graph without any changes, and hence is the graph in which one of the nutrient is limiting.   

In the graph for +P, there is no rise in biomass and the graph resembles the control. This shows that Phosphorous is not the first(initial) limiting nutrient, as its addition did not affect the biomass production.  

The +N graph shows a significant rise in biomass, indicating that N was indeed the initial limiting nutrient. On its addition, the biomass production increased.  

The +N and +P graph shows a higher biomass than +N and the control. This indicates, that while  N was established to be the first(initial) limiting reagent, addition of Phosphorous also significantly improves the biomass yield, and hence P must also have become a limiting nutrient once N was added and its limiting effect removed. That is the reason why +N+P graph is higher than just +N graph. If phosphorous would not have been limiting even on addition of N, then the +N+P graph would have resembled +N graph.