This is also a good ref on Hovmoller diagrams
Fedorov, Alexey V, and Jaclyn N Brown. “Equatorial Waves.” In Ocean Currents, Ed. J. H. Steele, S. A. Thorpe, and K. K. Turekian. Elsevier Science, 2010. http://people.earth.yale.edu/sites/default/files/files/Fedorov/50_Fedorov_EqWaves_Encyclopedia_2009.pdf.
This diagram is an idealized version of what would happen if the standing wave was closer to a pure sine wave without harmonics. From Federov, on the left is their ocean response to oscillatory winds in a shallow-water model, shown as a Hovmoller diagram of the thermocline depth anomalies on the equator. On the right is my simple LTE model with a single forcing cycle.
So Hovmoller is essentially plotting the spatiotemporal standing wave as a surface countour plot