This issue with Wundermap has, like many others, been going on for a long time, especially as the problem is being denied/ ignored by the Wundermap supremos.
The issue is this: when clicking on a site in Wundermap and then selecting GRAPH instead of CURRENT you are presented with a mini graph of temperature. So far so good, however if you go back to this same site and graph a few hours later you will often find that the graph has stopped at the time you previously accessed it.
If you look at these graphs regularly, as I do, this bug can drive you up the wall.