In both situations, you see a cyclical increase in pain before receiving the injection. This could be seen if the effect was wearing off; however, if you notice, there is a decrease in pain before the injection is even given. I believe this implies that something inherent about the injection is providing pain relief.
Furthermore, both curves follow the same path with an eventual decrease in pain, regardless of intervention.
Maybe an additional point - I am not questioning their pain, but fibromyalgia typically shows no abnormal lab results in addition to typically coexisting with some form of mental pathology. I don't think placebo being demonstrated is far off.