Hi Mcioci,
I personally would not diagram the argument, because I feel that it is easier to understand it in other ways. For example, you can diagram the argument in the following way:
Previously supported free trade in N.A.
some Not support free trade with these L.A. countries
I personally summed it up in this way:
There is at least one politician who was for that agreement, but who is not supporting this one.
Of course, I did not write this down. If I can comprehend a single, simple statement such as this without diagramming it, I don't see the point in diagramming it. For me, translating this statement into a conditional made it more difficult to understand.
Diagramming statements is a tool that should make the test easier for you. If diagramming this statement helped you understand the stimulus, then go ahead and do so!
More important than diagramming a statement is accurately reading it. For example it looks like in your diagram that you left out the time shift in the stimulus.