Oh good heavens--one would think that you don't know how to read a graph, or that you don't understand how to test claims empirically. Neither of those seem plausible, however. Is it that you want to make a rhetorical point so badly that you are content making incorrect statements as long as they're sort of in the right direction of a correct but more complex statement that you could make?

What the Laffer Curve states is that if you raise taxes and tax revenue goes down, raising them yet more will cause tax revenue to go down more.

This is eminently testable, as you very well should know.

That there is a theory is not the problem. The problem is that people hate testing economic theories empirically, and a global one like this is very hard to collect evidence for. It has approximately nothing to do with analytic philosophy, except that self-respecting empiricists should admit uncertainty when the models they create require observations that can't be made.