Each arm is one tidal constituent fitted to the Port of Olympia gauge. Its length is the amplitude, it turns at that constituent’s speed, and the pen’s height is their sum.
Paste a harmonic fit in the same shape as /api/tides/harmonics (constituents with name, amp_ft and phase_deg, plus z0_ft and optionally utide_defs), or choose the file. It’s for this visit only; reloading goes back to the gauge’s fit.