Fetch data from Google Sheets
Simple: specify a particular cell of a particular sheet as an odometer
Sophisticated: specify date and data-value columns from a particular sheet.

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Christopher Allen commented
My 'sophisticated' implementation was along the lines of one-row-per-day, but here's a...
More sophisticated: specify date, data-value and comment columns from a particular sheet (allow multiple rows per date).
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Christopher Allen commented
Motivation: I already track lots of stuff (e.g. pomodoros completed) in Google sheets, and having to manually transcribe data is annoying.
I gather that I might be able to do this using Zapier, but I aspire to complete more than 100 pomodoros a month so that will get expensive.
IFTTT ought to be able to do this, but it doesn't seem to be able to trigger based on changes to a sheet (or indeed any other triggers for Google Drive at all).
It might be possible to do this using Google App Script (to push data to Beeminder), but even if Ap Script will let me connect to external sites (which I am doubtful about) it would still be better to pull data rather than push it.