iSpindle Plotter is a self-hosted companion app for the iSpindel — an open-hardware floating hydrometer used by home brewers to track specific gravity through a fermentation. The app runs a small HTTP server on your phone that the iSpindel posts its readings to over your local Wi-Fi network, then stores, charts, and analyses those readings without ever sending your data to a third-party cloud service.

Pairing and configuration:

Auto-pair flow that joins the iSpindel's configuration access point, sets the phone as the HTTP target (Generic HTTP / selAPI=3), reads the device's existing cubic calibration polynomial off the firmware, and reconnects to your home network — without losing pre-existing fields like the calibration on subsequent saves.
mDNS-style hostname discovery so the iSpindel can find your phone again after either device's IP changes.
Pushes the app's own calibration polynomial back to the iSpindel when you re-calibrate.

Calibration:

Fit cubic / quadratic / linear polynomials from your own sucrose-dilution measurements.
Imports the iSpindel's pre-installed factory calibration on first pairing.
Setup guide with a sucrose-step-dilution recipe for 1.000–1.080 SG.

Plotting and analysis:

Tilt angle, temperature, specific gravity, and battery voltage charts with a 24h / 7d / 30d / All time-window switcher.
Potential-alcohol secondary axis on the SG chart.
Robust phase classifier (Lag / Active / Slowing / Conditioning / Stuck / Cold crash) using a modified-Gompertz Levenberg-Marquardt fit (Zwietering et al. 1990), with fallback to an attenuation-prior when the data is too early to constrain the asymptote.
Predicted final gravity overlaid on the chart with a tapering uncertainty band derived from short-time noise estimation and the calibration polynomial's residual.
Battery runway estimate: linear discharge fit projecting time-to-cutoff at the current sample rate.
Trim-before-date-and-time to drop pre-fermentation noise; CSV export via the system file picker.

Privacy:

No analytics, no telemetry, no third-party network connections.
Data is held only on your phone and only sent to your iSpindel's local IP for configuration writes.
The app's HTTP server listens on the phone's local network address only.

Hardware: built and tested with an MTB iSpindel PCB 4.0; should work with any iSpindel firmware that supports the Generic HTTP target.