Thanks for your hints.
It now works fine with the temp+humidity sensor, it shows correctly in two charts. However, when I add a second temp sensor, it writes to the same chart as the first one. I then used the same write key for both sensors. I did separate update times like for the first sensor every 60 second, for the second one every 327 seconds so as to minimize update collisions. This did not help. Nothing is written to the third chart that I created, although the sensor web page says that it is connected to thingspeak, So it seems that I need to use a second API write key for the second temp sensor. However, I am not sure if one channel allows for several write keys. Also, I can' find how to create a second channel within my present account. Maybe one has to start with a second account.
It seems that the Thingspeak channel system can not separate between sensors if they have the same function, although their MAC adresses and device IDs are distinct. How can I make the channel system differentiate between two sensors of the same type within the same channel?
PS I created a second account with a new channel and a separate write API key for the second temp sensor. This works, but it would be better to be able to have several sensors of the same type but with different device IDs within one channel. The problem seems to be that one can not assign a specific device ID to a specific field. Edit: The second account excluded the first in spite of the two accounts having different user names and email addresses (but the same password). When I type in the URL for the first channel https://thingspeak.com/channels/xxxxxx/private_show I get the message that the page can not be found. This problem was only solved with using different browsers for the two accounts (safari and chrome on Iphone).
