Interpretation of input impedance graph of a filter
Yes, you have plotted the input impedance of your circuit.

The solid line is the magnitude, and the dashed line is the phase (relative to the right-hand scale) of the impedance. The phase might be 180 degrees off due to the definition of I(V1). If I(V1) is the current going in to the positive terminal of the supply you should use -I(V1) to get the current going out into the circuit.  

Having the magnitude on a dB scale is not especially helpful -- better to figure out how to plot it on a linear scale. But this is not necessarily easy in LTSpice, it took me a while to figure out how to do it when I was doing something similar the other day, and I can't remember the correct incantations now.