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On the parallel interface of my computer I've plugged in a small device, that measures the air pressure. Every 10 seconds the software reads the actual value out of the measure device and stores it. After 2 minutes, the software calculates the mean value and stores the result into a ring buffer. This ring buffer can store exactly 720 entries, what's enough for 24 hours.
You can find a detailed description at Linux Focus. There you can download a basic software to get out something of your sensor. Based on this software, I developed a real daemon and added some features.
Because it's a real daemon now, it detaches itself from the console and writes all messages to your syslog file. The program is able to store all measured values in a file. The values are stored in binary notation. This makes it faster and needs less space on disk. The original behavior on a network is unchanged. Hence the client is also unchanged. Now you are able to look down to the first measured value and display it.
In case your computer is not running 24/7, you may have gaps in the curve. The program recognizes this and makes the gaps visible. This is possible, because we store a time stamp to every measured value in the file.
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