Works fine on first access then when it refreshes I sometin=mes get just a 0 in the upper left corner and the rest is blank.
Using Firefox 57.0.2.
Reloading gets the regualr screen back until the next update.
This zero appears if there are no new spots to moment of the next automatic page reload. But the script seems to be working. Because if a new spot appears in the interval between two automatic reloads of the page, zero disappears and the cluster is displayed again normally.