I am testing Google Sheets spreadsheet which holds granular heart rate data. This is non trivial task because sensors provide datapoint for every 1 or 5 seconds depending on device and several years of data add up to hundreds of thousands if not millions data points.
The advantage of having such data in spreadsheet is that one can use custom metrics and cross reference different data sources. Also, only you have access to all your data. It is stored on Google Drive, but that's the extent of use of 3rd parties.
The spreadsheet currently supports Polar ProTrainer HRM files, and CSV exports which can be collected from any ANT+ compatible bluetooth chest strap, and Polar H7. It can be extended to support Garmin and other sensors.
Please let me know via message or reply if you would be interested in taking a look at early version. I intend to make it open source once it is a bit more tested. Attaching couple screenshots.