iSpindle Plotter — Open Source Productivity App

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.