plot tab: fix y-axis scaling, add Visible/Used Sats modes, right-click menu

- Fix _nice_ticks: use floor for first tick and ceil for last tick so the
  tick range always fully brackets the data; data was previously clipped
  when the maximum fell between two nice tick values
- Add 5% headroom padding so data does not sit flush against plot edges
- Add 'Visible Sats' and 'Used Sats' y-axis modes (global, like pDOP)
- Right-click context menu on plot canvas: Save as PNG and Copy to clipboard

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/plot_widget.py

@@ -34,7 +34,9 @@ from PyQt5.QtCore import Qt, QPointF, QRectF, QThread, pyqtSignal
 
 from data_model import ReceiverManager
 
-_Y_OPTIONS = ['C/N₀', 'Azimuth', 'Elevation', 'pDOP']
+_Y_OPTIONS = ['C/N₀', 'Azimuth', 'Elevation', 'pDOP', 'Visible Sats', 'Used Sats']
+
+_GLOBAL_MODES = {'pDOP', 'Visible Sats', 'Used Sats'}
 _COLORS    = ['#2196F3', '#F44336', '#4CAF50', '#FF9800',
               '#9C27B0', '#00BCD4', '#FFEB3B', '#795548']
 
@@ -61,9 +63,10 @@ def _nice_ticks(lo: float, hi: float, n: int = 5) -> list:
         if base * m >= raw:
             step = base * m
             break
-    first = math.ceil(lo / step) * step
+    first = math.floor(lo / step) * step   # ≤ lo  — data never clipped at bottom
+    last  = math.ceil(hi  / step) * step   # ≥ hi  — data never clipped at top
     ticks, v = [], first
-    while v <= hi + step * 0.01:
+    while v <= last + step * 0.01:
         ticks.append(v)
         v += step
     return ticks
@@ -148,7 +151,9 @@ class PlotRenderThread(QThread):
         if ts_max - ts_min < 1:
             ts_min = ts_max - 1.0
 
-        v_ticks = _nice_ticks(v_min, v_max)
+        # Add 5 % headroom so data never sits right on the plot edge
+        pad    = (v_max - v_min) * 0.05 if v_max > v_min else 0.5
+        v_ticks = _nice_ticks(v_min - pad, v_max + pad)
         v_min, v_max = v_ticks[0], v_ticks[-1]
         if v_max - v_min < 1e-9:
             v_max = v_min + 1.0
@@ -268,6 +273,22 @@ class _Canvas(QWidget):
         super().resizeEvent(event)
         self.resized.emit(self.width(), self.height())
 
+    def contextMenuEvent(self, event):
+        from PyQt5.QtWidgets import QMenu, QFileDialog
+        if not self._image or self._image.isNull():
+            return
+        menu = QMenu(self)
+        act_save = menu.addAction("Save as PNG…")
+        act_copy = menu.addAction("Copy to clipboard")
+        action = menu.exec_(self.mapToGlobal(event.pos()))
+        if action == act_save:
+            path, _ = QFileDialog.getSaveFileName(
+                self, "Save plot", "", "PNG (*.png)")
+            if path:
+                self._image.save(path)
+        elif action == act_copy:
+            QApplication.clipboard().setImage(self._image)
+
 
 # ── Single configurable plot ──────────────────────────────────────────────────
 
@@ -360,7 +381,7 @@ class SinglePlotWidget(QWidget):
             self._y_cb.blockSignals(True)
             self._y_cb.setCurrentText(s['y_axis'])
             self._y_cb.blockSignals(False)
-            self._val_cb.setEnabled(s['y_axis'] != 'pDOP')
+            self._val_cb.setEnabled(s['y_axis'] not in _GLOBAL_MODES)
         if 'history' in s:
             self._hist_spin.blockSignals(True)
             self._hist_spin.setValue(s['history'])
@@ -424,8 +445,7 @@ class SinglePlotWidget(QWidget):
         self._submit_render()
 
     def _on_y_changed(self, _text: str):
-        is_pdop = (_text == 'pDOP')
+        self._val_cb.setEnabled(_text not in _GLOBAL_MODES)
-        self._val_cb.setEnabled(not is_pdop)
         for rid in list(self._data):
             n = self._hist_spin.value()
             self._data[rid] = [deque(maxlen=n), deque(maxlen=n)]
@@ -470,6 +490,13 @@ class SinglePlotWidget(QWidget):
         if rid not in selected:
             return
         y_axis = self._y_cb.currentText()
+
+        if y_axis == 'Visible Sats':
+            self._push_point(rid, time.time(), len(sats))
+            return
+        if y_axis == 'Used Sats':
+            self._push_point(rid, time.time(), sum(1 for s in sats if s.used_in_fix))
+            return
         if y_axis == 'pDOP':
             return