Saving a lot of temperature history

Hi,

At the moment, my Arduino Mega is uploading the temperature every 30 seconds to Cosm. This is great, but now I would like to save and graph all the data myself.
Could someone give me some suggestions on how I can store multiple days worth of readings?
If possible, without needing a computer online constantly.

Thanks.

Use a serial logger, like this:

I'd completely forgotten about the SD card reader on the ethernet shield - I'll use that.

Thanks for your answer