refactored graph for clear time based data display

docs/GUI_DESIGN.md

@@ -92,18 +92,43 @@ Located in the right column, below the info cards. Hidden for CC/CV/CR/CP modes.
 - Window size is persisted between sessions (saved in settings)
 - Minimum window size: 400×400 px
 - Combined mode: voltage scale on left axis, current and power scales on right axis (color-coded)
+- **X axis is time-based**, not sample-index based. Points are positioned by timestamp within an
+  explicit time domain, and the axis is labelled underneath:
+  - Roll: labels are relative to now (`-1h02m`, `-2m05s`, `-45s`, `0`)
+  - Infinite: labels are wall-clock (`HH:MM:SS`)
+  - Full labels are drawn when the plot is at least 320 px wide; narrower plots are labelled at
+    their ends only. Split ↕ labels the bottom sub-chart only (all sub-charts share one domain).
+- Traces are **min/max decimated** to at most two points per horizontal pixel. The full time span
+  is always drawn — there is no cap on how many samples a plot may cover — and single-sample
+  transients survive decimation.
+- A pause longer than 5 poll intervals (minimum 1.5 s) **breaks the polyline**, so a disconnect or
+  a paused log renders as a gap instead of a straight line. An isolated reading between two gaps is
+  drawn as a dot.
 - Time mode controls (bottom toolbar row, left-aligned):
-  - **Mode toggle button** shows current mode: `⟳ Roll` or `∞ Infinite`; click to switch
-  - **Roll mode**: rolling window of the last N seconds; time window input + "Set" button are shown
-  - **Infinite mode**: all data since app start or last Clear; time window input and "Set" are hidden
-  - **Clear button** (Infinite mode only): resets graph display start time to now; does NOT delete samples
+  - **Mode toggle button** shows current mode: `⟳ Roll` or `∞ Infinite`; click to switch.
+    Switching is a **view-only** change: it never adds to or removes from the sample buffer, so it
+    is safe to toggle mid-run.
+  - **Roll mode**: the last N seconds; time window input + "Set" button are shown. The domain is the
+    window itself, so a partly-filled window draws on the right-hand portion of the canvas rather
+    than stretching to fill it. The window is clamped to the history retention setting.
+  - **Infinite mode**: everything retained in the buffer since the last Clear; the window input and
+    "Set" are hidden.
+  - **Clear button** (both modes): sets the graph's view epoch to now. Samples before the epoch are
+    hidden from the graph in **both** modes, but are **not** deleted — a CSV export still contains
+    them. Pressing Clear is the only action that hides recorded data from the graph.
+  - **Buffer label**: wall-clock span currently held in the buffer, so a window wider than the
+    available history is visibly explained rather than silently ignored.
 - Hide/show toggle button
-- Graph data is independent of CSV export (export uses all collected samples)
+- The graph and CSV export read the **same** buffer. Retention (Settings → Application → History
+  retention, default 86400 s = 24 h) bounds both together: the graph can never show something an
+  export would miss, and vice versa. Clearing the samples buffer also resets the graph's view epoch.
+  The 24 h default is sized for long CAP runs such as a car battery discharge.
 
 ### 6. Samples Panel
 - Sample count
 - Last sample summary (V/I/P values)
-- Clear button (clears all samples)
+- Clear button (deletes all samples — unlike the chart's Clear, this discards data that would
+  otherwise be exportable; it also resets the chart's view epoch)
 - Export button (saves CSV with columns: timestamp, voltage, current, power, resistance, mode)
 
 ### 7. Connection Panel
@@ -160,6 +185,12 @@ Responsive card-based layout. On wide windows (≥720px), cards are arranged in
    - Language (dropdown)
    - Poll interval (dropdown: 50/100/200/500/1000/2000 ms; helper text below)
    - Auto-connect to first EL15 (toggle)
+   - History retention (text input + "Set"; seconds, clamped to 60–86400, default 86400 = 24 h;
+     helper text below). Bounds the shared sample buffer used by both the graph and CSV export.
+     Shrinking it trims the buffer immediately and pulls the Roll window in with it.
+     A hard backstop of 500 000 samples (~32 MB) also applies: it sits just above 24 h at the
+     default 200 ms poll, so a *faster* poll reaches the ceiling first and retains proportionally
+     less wall-clock time (at 50 ms, roughly 7 h).
 
 2. **SCPI Server**
    - Enable SCPI server (toggle)

el15-app/locales/en.json

@@ -82,12 +82,15 @@
   "graph.infinite": "Infinite",
   "graph.time_window": "Window",
   "graph.clear": "Clear",
+  "graph.buffer": "Buffer",
   "settings.scpi_section": "SCPI Server",
   "settings.firmware_section": "Maintenance",
   "settings.about_section": "About",
   "settings.repository": "Repository:",
   "settings.card.application": "Application",
   "settings.poll_hint": "Lower values increase responsiveness but may increase device traffic.",
+  "settings.retention": "History retention",
+  "settings.retention_hint": "How long samples are kept for the graph and CSV export.",
   "settings.scpi_port_hint": "Used by external tools to control the load over SCPI.",
   "settings.maintenance_desc": "Device must be in DFU mode for firmware update. See Firmware Update page for instructions.",
   "settings.auto_saved": "Settings are saved automatically.",

el15-app/locales/es.json

@@ -82,12 +82,15 @@
   "graph.infinite": "Infinito",
   "graph.time_window": "Ventana",
   "graph.clear": "Limpiar",
+  "graph.buffer": "Búfer",
   "settings.scpi_section": "Servidor SCPI",
   "settings.firmware_section": "Mantenimiento",
   "settings.about_section": "Acerca de",
   "settings.repository": "Repositorio:",
   "settings.card.application": "Aplicación",
   "settings.poll_hint": "Valores más bajos mejoran la respuesta pero pueden aumentar el tráfico al dispositivo.",
+  "settings.retention": "Retención del historial",
+  "settings.retention_hint": "Cuánto tiempo se conservan las muestras para el gráfico y la exportación CSV.",
   "settings.scpi_port_hint": "Usado por herramientas externas para controlar la carga por SCPI.",
   "settings.maintenance_desc": "El dispositivo debe estar en modo DFU para actualizar firmware. Vea la página de actualización para instrucciones.",
   "settings.auto_saved": "Los ajustes se guardan automáticamente.",

el15-app/locales/hi.json

@@ -82,12 +82,15 @@
   "graph.infinite": "अनंत",
   "graph.time_window": "विंडो",
   "graph.clear": "साफ़ करें",
+  "graph.buffer": "बफ़र",
   "settings.scpi_section": "SCPI सर्वर",
   "settings.firmware_section": "रखरखाव",
   "settings.about_section": "के बारे में",
   "settings.repository": "रिपॉजिटरी:",
   "settings.card.application": "एप्लिकेशन",
   "settings.poll_hint": "कम मान अधिक त्वरित प्रतिक्रिया देते हैं लेकिन डिवाइस का ट्रैफिक बढ़ा सकते हैं।",
+  "settings.retention": "इतिहास प्रतिधारण",
+  "settings.retention_hint": "ग्राफ़ और CSV निर्यात के लिए नमूने कितने समय तक रखे जाते हैं।",
   "settings.scpi_port_hint": "SCPI के माध्यम से लोड को नियंत्रित करने के लिए बाहरी उपकरणों द्वारा उपयोग किया जाता है।",
   "settings.maintenance_desc": "फर्मवेयर अपडेट के लिए डिवाइस DFU मोड में होना चाहिए। निर्देशों के लिए फर्मवेयर अपडेट पेज देखें।",
   "settings.auto_saved": "सेटिंग्स स्वचालित रूप से सहेजी जाती हैं।",

el15-app/locales/ru.json

@@ -82,12 +82,15 @@
   "graph.infinite": "Полностью",
   "graph.time_window": "Окно",
   "graph.clear": "Очистить",
+  "graph.buffer": "Буфер",
   "settings.scpi_section": "Сервер SCPI",
   "settings.firmware_section": "Обслуживание",
   "settings.about_section": "О программе",
   "settings.repository": "Репозиторий:",
   "settings.card.application": "Приложение",
   "settings.poll_hint": "Меньшие значения повышают отзывчивость, но увеличивают нагрузку на устройство.",
+  "settings.retention": "Хранение истории",
+  "settings.retention_hint": "Сколько времени хранятся отсчёты для графика и экспорта в CSV.",
   "settings.scpi_port_hint": "Используется внешними инструментами для управления нагрузкой через SCPI.",
   "settings.maintenance_desc": "Устройство должно быть в режиме DFU для обновления прошивки. Инструкции на странице обновления.",
   "settings.auto_saved": "Настройки сохраняются автоматически.",

el15-app/locales/zh.json

@@ -82,12 +82,15 @@
   "graph.infinite": "无限",
   "graph.time_window": "窗口",
   "graph.clear": "清除",
+  "graph.buffer": "缓冲",
   "settings.scpi_section": "SCPI 服务器",
   "settings.firmware_section": "维护",
   "settings.about_section": "关于",
   "settings.repository": "代码仓库:",
   "settings.card.application": "应用程序",
   "settings.poll_hint": "较小的值提高响应速度，但可能增加设备通信负担。",
+  "settings.retention": "历史保留时长",
+  "settings.retention_hint": "采样数据为图表和 CSV 导出保留的时长。",
   "settings.scpi_port_hint": "外部工具通过 SCPI 控制负载时使用此端口。",
   "settings.maintenance_desc": "设备需处于 DFU 模式才能更新固件。请参阅固件更新页面的说明。",
   "settings.auto_saved": "设置会自动保存。",