To measure the temperature I used an LM75A which was lying around, used an AVR to read the sensor over I2C and send the measured value as text over the serial port. An USB serial converter then makes the connection to the computer.
A shell script on the ALIX then reads the data from serial port and uploads a temperature value to golem every 15 minutes.
The whole code for this project can be found in my repository.
When using a token for the uploads Golem allows to see the measurements of the last week as a nice graph. Too bad they don’t create token pairs, a private token for the uploads and a public read-only token for viewing the data. Then I could send the Chart URL with the public token to friends without the risk that somebody can insert wrong data into my chart.
Update 2016-05-10 17:12:
I have my chart now online. If you also want to publish your own chart you only need the following two files: chart.php and data.php. Edit data.php and add the token, then view chart.php in your browser.