I've just release a new, improved version of the W1209 Data Logging Thermostat: now the data logger registers the following data:

• relay activation cycles per log interval
• relay duty cycle
• lowest, and highest temperature in the cycle

Used properly, this data can be used to improve the control behavior (heating power), or for getting clues on the effects of heat flow & thermal mass, sensor location, or quality of the insulation:

Check out the README on Github for features, source, and instructions!