Temperature raise inside a case
Free air convection assumes that there is enough air around the part (or in the room) that the ambient temperature of the air would not be affected by the heat from the part. 

If the part is in an enclosure then you introduce a thermal barrier between the air in the room and the enclosure and heat the enclosure air up.  So interpreting the graph, the air in your enclosure will probably be more than 43C while running at 2A because of the added thermal resistance. The temperature range of the part is 125C (which you probably wouldn't the part or other parts in you enclosure to reach but it will still operate). 

One thing you will want to avoid is thermal runaway where the heat of the part increases the internal resistance and leads to more heat being generated by the part until you have a meltdown. This will be determined by the thermal resistance of the enclosure. The a good way to determine what the internal temperature would be to experimentally verify it with the design in the enclosure or find the max power dissipation of the design and put a dummy load in the enclosure and measure the temperature.