Indigo and Cosm.com

Xively will allow you to generate a PNG using its API for any timespan (provided you don't breach their limits on datapoints, which are pretty generous for home automation purposes):

See in particular these links:

https://xively.com/dev/docs/api/data/re ... atastream/

https://xively.com/dev/docs/api/quick_r ... resources/

https://xively.com/dev/docs/api/quick_r ... ical_data/

An example URL for the image would be something like:

https://api.xively.com/v2/feeds/[FEED_ID_GOES_HERE]/datastreams/DATASTREAM_ID_GOES_HERE.png?scale=manual&min=0&max=30&b=true&l=%22Temp%20in%20C%22&t=%22Title%20Here%22&g=true&duration=1day&w=600&h=250

I'm completely new to this tonight, but have managed to get graphs up and running and embedded in Indigo with a quick couple of hours' reading.