The top graph shows the impedance values (electrical resistance for AC current) of the M80 speaker when driven at the frequency designated in the horizontal axis. You see from this graph that the actual impedance value of the M80 never goes below ~4 ohms (the minimum in this case is between 100-500Hz). This shows that the "nominal" 4-ohm rating of the M80 is very honest -- many "4-ohm" speakers, including my own Hales, actually go down to 3 ohms and 2 ohms in certain frequencies, presenting a tough load for the power amp (my 45TX receiver still doesn't have a problem, though).
The bottom graph shows the "phase angle" of the AC current flowing into the M80 when driven by sine-wave voltage of the designated frequencies. Zero phase angle means that the current is in-phase with the voltage, showing that the speaker is behaving as a simple resistor at that frequency (a resistive load). When the phase angle swings positively, that means that the current flow is "delayed" as compared to the voltage swing, an indication of a "inductive" load, which most speakers behave like because the voice coil is in fact an inductor. A negative phase angle, in contrast, is a "capacitive" load (a condenser-like load), which is sometimes tough on the amplifier when the negative phase deviation is large. Anyway, the M80 shows very mild phase angles, pretty much always between 0-45 degrees over the entire frequency range. Again, this is one of the most benign loads I've seen.
Bottom line: From the amp's perspective. the M80 is one of the easiest 4-ohm speakers to drive.
