Voltage from IO pin at max current
According this datasheet

The Absolute Maximum Ratings for DC current per IO pin is indeed 40 mA.



However, you should not operate a device at its Absolute Maximum Ratings, but at their Recommended Operation conditions.

Although the output current is not explicitly stated, there are tiny notes in the recommended operating conditions:





The reliable sink/source current at 3V supply is only 5 mA.
So, I'd suggest using a transistor instead of driving the LED directly by the uP.  


  Is there a way to figure out what the voltage would be at this current? 


Yes, by testing and measuring it yourself.


  Also, why didn't the designers put those values in the graph?


See, answer above. They still exceeded the 5 mA, but the tiny notes say the data is not guaranteed.