The free version of Thingspeak has a limitation of maximum one post (logging) per 15 seconds per channel.
Each channel can contain up to 8 fields.
Currently, you can only show 1 field (plot) in each chart (would be nice if it was possible to show eg. indoor and outdoor temperature in the same chart).
Have a look here for examples and tutorials:
Direct link to GitHub with the current arduino example:
Link to my test channel, containing outdoor temperature readings: