This tool was made because NEST only displays the last 10 days of energy usage in their app and webclient. For me this was a pretty big limitation, rendering the whole idea of getting a detailed overview useless. Based on how NEST interacts with their customers, I don't expect them to fix this problem any time soon. That's why I decided to make the tool myself.
The tool can be used with historical data that is available here:
Request an archive with all data and wait for it to be processed (this can take a long time). Then extract the data from the zip file and simply find the month that you'd like to check out. Then drag the "year"-"month"-summary json file on the site, and you will get the overview of that month.
Please note that all parsing is done on client-side. No reports are uploaded to a server!