Calibrate EQ Using Speaker SPL Graph
Well, calibrating EQ for home speakers is not a common practice. However it seems that the easiest and cheapest way to do it is just play some music through them and record the output. Then you could use one of the free audio software (e.g. Audacity) to import the recorded audio and get the frequency spectrum graph. Using this graph you might get a general idea of salient frequencies.

And now regarding the graph you see.. You're right, usually the frequency response is represented with one line in dB/Hz units. However in your case there are actually 6 different graphs in one picture:


Regular frequency response (one of the 3 bottom lines with no comments near it)
Frequency response of 2nd and 3d harmonics distortion (the ones wit 2nd HD and 3rd HD comments near them)
What seems to be a phase response (the 3 graphs on top). This one is pretty unusual, especially for home speakers.