Depending on what a client is willing to do, we can basically flatten the response of any room, widen the sweet spot and even the decay times. This is hard particularly in small rooms, but it can be done. For rented spaces it's a little more limited, as you really can not do the serious construction to fully optimize a room, but there is a lot you can do.
In general EKG graphs are not what we're looking for (unless the person is dead--then that basic flat look is a good thing for audio). Can you e-mail me your graph and a drawing of the room? Even better would be to download the application form and fax it to me along with the graph. The form can be downloaded here. From that I could give you a very good idea of what could be done without the use of permanently installed devices.