I used Claud to help me troubleshoot and craft this post, but I am a real person w/ this issue 
I’m working through the new custom zones feature (November 2025) and have two related questions about gaps in fitness page chart support.
Time-in-custom-zones as a selectable plot on /fitness page
Custom zones work well on activity detail pages, the totals page, and workout prescription. However, when building charts on the /fitness page via Options → Custom Charts → Add Chart → Add Plot, only built-in zone time plots (HR1, HR2, Z1, Z2 etc.) appear as options — custom zones are not selectable.
Is time-in-custom-zones currently available as a plot in the fitness page chart builder? If so, what is the correct search term or field name? If not, is it planned?
The built-in zones are not a viable workaround for anyone whose custom zones use different boundaries — particularly when zones are anchored to lactate-tested aerobic and anaerobic thresholds that don’t align with the default zone splits. Manually mirroring custom zone boundaries into the built-in zone editor defeats the purpose of having custom zones entirely.
Custom zone time fields in the new Formula builder
The custom formula feature (2 March 2026) looks potentially useful as a workaround here. The examples show built-in zone fields like time_in_hr_s1, time_in_hr_s2 etc. working in formulas. Are custom zone time fields accessible as formula variables?
Why this matters
The core value of custom zones is enabling zone-based analysis that the built-in system can’t provide. Without fitness page chart support, custom zones are useful at the activity level but invisible for longitudinal trend analysis — which is precisely where zone distribution tracking is most valuable.
Thanks - TJ