This app was mentioned in 4 comments, with an average of 1.25 upvotes
Tasker + nest plugin + WatchMaker works for me.
Nest plugin https://play.google.com/store/apps/details?id=com.t3hh4xx0r.nest_plugin
WatchMaker let's you place tasker variables on the face
https://play.google.com/store/apps/details?id=slide.watchFrenzy.premium
In terms of storing data... I imagine that's relating to any application that uses the API. Once the data is exported, it's not the app storing the data any longer.
That being said - it does kinda eat into how useful this would be as a user would have to remember to export it at least every week, and merge their files removing any duplicates. It would be a hassle for sure. Oh well...
Right now I'm already doing this via a Smart Thermostat Plugin (https://play.google.com/store/apps/details?id=com.t3hh4xx0r.nest_plugin) for Tasker, which polls every 10 minutes and then uploads the data to Xively (a web data storage and graphing service),which gives me this: http://i.imgur.com/ZyYfAPI.png
But your app is far more elegant.
This Tasker/Locale plugin for Nest works great. I've found it to work better for my needs than both IFTTT and Google Now's Nest controls. Last I checked neither could query the current temperature setting of the thermostat which this plugin can do. I like Tasker being able to know what my thermostat is set to (in case somebody at home manually adjusted it). I also have Tasker dynamically adjust the temp based on what the outside temperature is, instead of the static threshold that Nest's 'Away' mode uses. As an example: if the temperature is 95° outside I don't mind away mode kicking my AC on if inside goes above 80°, but when it's 112° outside I don't want my AC wasting the energy to keep the inside at 80°, so Tasker sets it to something higher like 88°. Last I checked my Smartthings hub did not have Nest support.
https://play.google.com/store/apps/details?id=com.t3hh4xx0r.nest_plugin
Use this: https://play.google.com/store/apps/details?id=com.t3hh4xx0r.nest_plugin&hl=en_GB
You could also do this with IFTTT :)