請問縱波如何作圖?

If you want to draw a displacement-distance graph for a longitudinal wave, the following points is worth noting:
(i) points at compression and rarefaction have zero displacement;
(ii) points to the right of the mean position have +ve displacement;
(iii) points to the left of the mean position have -ve dispalcement;

The following web-page illustrates how to draw such a graph"

http://sciencecity.oupchina.com.hk/npaw/student/chi/supplementary/1c-graph.asp

You could also play with the animation in the following web-page, which draws out the displacement-time or velocity-time of a particle in a longitudinal wave:

http://homepages.ius.edu/kforinas/W/waves/Longitudinal.html