Thanks for coming back on it!
I think the whole thing works and I made a silly mistake while reading the results. I also learned something new about our indoor climate perhaps.
First:
I think the apparatus works. My reference, the analog meter, showed the same value and has continued to do so. That means my circuit and the analog meter either both work or have the same kind of strange issue (not likely).
Second: (my reading mistake)
I said 12:34, because that is what I read on the graph and at 12:34 everything was static and quiet. HOWEVER I didn't think about the time zone when writing my script to convert the timestamps to JSON data. The timestamp is CET and 12:34 is in reality 13:34.
At 13:34 my wife came home after having run 15km in the Munich Winterlauf Series.
Needless to say the door to the stairwells was open for a while and she was wet and still perspired after having run in the wet weather.
We then closed the door and no windows where opened for a couple of hours. Two hours later (15:39 CET) we vented the flat again and the humidity dropped again.
I have time zone corrected the conversion tool and uploaded the latest data as illustration.
Same place: http://wohnung.se/arduino/hygrometer/data.html
Thanks again for all the help and friendly support!
Anders