Apple Health includes caffeine data which is used by some caffeine logging apps including the one I use, RECaf. Caffeine isn’t currently available as a Beeminder Apple Health data source, though.
Does anyone here beemind caffeine via Apple Health? What would be the best way of automating this? I’m not averse to using IFTTT, Shortcuts, or a bit of scripting if necessary.
(I can’t help with the question but I’ve got a bit curious. My reply can be considered a bit out of topic, hopefully it doesn’t distract the conversation.)
Is there a particular reason that you’d like to (or are already) logging caffeine at the milligram prrecision (which seems to be Apple Health default)?
I myself have a “no-caffeine-days goal” that’s set to 5 days/week without caffeine. This seems enough for me since I rarely drink more than 2 cups (~330ml) a day and I always make them at the same intensity. This is enough for me, since it helps me avoid mood and sleep related negative effects of caffeine.
When I read your question, and having assumed you have a similar goal to mine, I got thinking whether I’m underestimating the caffeine sensitivity of the human body 
Wow, just one day without caffeine seems impossible for me, which I guess points to my concern that I’ve become dependent.
I don’t know how accurate RECaf or any of the other apps are, but I’m trying to get more insight into my caffeine consumption patterns, especially as I brew coffee and tea (either black or green) at home as well as drink different kinds of coffee in cafes. I find RECaf has useful warnings/reminders when I log caffeine after a set time of day, or more than average. So I’m developing a bit more mindfulness of my caffeine consumption than just using my current method of logging cups of coffee directly in beeminder,
For now, my solution is an iOS Shortcut that takes a sum of all my caffeine intake for the day and puts this in an email to bot@beeminder.com, and an automation to run this shortcut every evening.