But these are graphs, not numbers! What do you think Diego and Mai are thinking?

Answer:
Diego and Mai are thinking of the slopes of the graphs!
Step-by-step explanation:
The slopes of the graphs are as follows:
Right: Negative slope
Center: Zero slope
Left: Positive slope
Therefore, you can conclude that the graphs are organized in order of slope.
(Let me know if you need more explanation as to why we know the slopes are positive, negative, zero, etc.)