This example first finds the minimum value for each of the 24x24
long/lat positions, and then shows you the 20 lowest of those
on a map.  The markers are numbered 1-20, and you can mouse over
the markers to see the date & value, and you can click on the 
markers to see all the data for that marker (in a time-series plot).

Scroll down below the map, and you'll see bar charts for the
20 values, to facilitate easy comparison.