Your graph is generally correct, but the trick is going from the graph to a specific unit or procedure that will achieve safe drinking water. You probably should include UV filtration, which is becoming popular with wilderness users. Heavy metals could be a consideration in some urban situations, but are not much of a hazard in wilderness.

Selecting a treatment method is actually a fairly complex process, and many factors should be considered in making a decision. Your graph is a good beginning, but only a beginning.