What are the values on the x-axis when I plot the impulse response of a lowpass filter? I am trying to develop a filtering routine, using Java, and I am using the windowing method. 

From my studies, I was told that if I make a plot of the impulse response of an ideal lowpass filter, I would get a SINC graph. In this situation, what am I plotting against (i.e., what values are on the X-Axis?)?
For digital filters the x-axis is samples.