diff --git a/rfg_vna_viewer.py b/rfg_vna_viewer.py
index 6949cd0..c55b2cf 100644
--- a/rfg_vna_viewer.py
+++ b/rfg_vna_viewer.py
@@ -202,13 +202,16 @@ def build_gui():
     p_norm.setXLink(p_raw)
     c_norm_amp = p_norm.plot(pen=pg.mkPen((80, 120, 255), width=1))
 
-    # Row 3: normalized phase
+    # Row 3: normalized phase + linear fit + deviation
     p_ph = win.addPlot(row=3, col=0, title="Нормированная фаза arg(S)")
     p_ph.showGrid(x=True, y=True, alpha=0.3)
     p_ph.setLabel("bottom", "Частота", units="ГГц")
     p_ph.setLabel("left", "Фаза", units="рад")
     p_ph.setXLink(p_raw)
-    c_ph = p_ph.plot(pen=pg.mkPen((230, 180, 40), width=1))
+    p_ph.addLegend(offset=(10, 10))
+    c_ph = p_ph.plot(pen=pg.mkPen((230, 180, 40), width=1), name="Фаза")
+    c_ph_line = p_ph.plot(pen=pg.mkPen((180, 180, 180), width=1, style=QtCore.Qt.DashLine), name="Лин. прибл.")
+    c_ph_dev = p_ph.plot(pen=pg.mkPen((255, 100, 200), width=1), name="Отклонение")
 
     # Row 4: FFT distance
     p_fft = win.addPlot(row=4, col=0, title="FFT — расстояние")
@@ -217,15 +220,17 @@ def build_gui():
     p_fft.setLabel("left", "Магнитуда", units="дБ")
     c_fft = p_fft.plot(pen=pg.mkPen((60, 200, 80), width=1))
 
-    return app, win, (c_main_amp, c_ref_amp, c_norm_ch1, c_norm_ch2, c_norm_amp, c_ph, c_fft)
+    plots = [p_raw, p_ch, p_norm, p_ph, p_fft]
+    curves = (c_main_amp, c_ref_amp, c_norm_ch1, c_norm_ch2, c_norm_amp, c_ph, c_ph_line, c_ph_dev, c_fft)
+    return app, win, curves, plots
 
 
 # ---------------------------------------------------------------------------
 # Update loop
 # ---------------------------------------------------------------------------
-def make_update(reader, parser, assembler, curves):
-    c_main_amp, c_ref_amp, c_norm_ch1, c_norm_ch2, c_norm_amp, c_ph, c_fft = curves
-    state = {"ref_phase_first": None}
+def make_update(reader, parser, assembler, curves, plots):
+    c_main_amp, c_ref_amp, c_norm_ch1, c_norm_ch2, c_norm_amp, c_ph, c_ph_line, c_ph_dev, c_fft = curves
+    state = {"ref_phase_first": None, "axes_locked": False}
     queue = deque(maxlen=64)
 
     def update():
@@ -269,9 +274,21 @@ def make_update(reader, parser, assembler, curves):
         c_norm_amp.setData(freqs_ghz, norm_amp)
         c_ph.setData(freqs_ghz, phase)
 
+        # Linear fit (line through first and last point) + deviation
+        line = np.linspace(phase[0], phase[-1], len(phase))
+        deviation = phase - line
+        c_ph_line.setData(freqs_ghz, line)
+        c_ph_dev.setData(freqs_ghz, deviation)
+
         fft_db = 20.0 * np.log10(fft_mag + 1e-12)
         c_fft.setData(fft_dist, fft_db)
 
+        # Lock axes after first sweep
+        if not state["axes_locked"]:
+            state["axes_locked"] = True
+            for p in plots:
+                p.enableAutoRange(False)
+
     return update
 
 
@@ -283,9 +300,9 @@ def main():
     reader = DataReader(path)
     parser = LegacyBinaryParser(batch_events=True)
     assembler = SweepAssembler(fancy=False, apply_inversion=False)
-    app, win, curves = build_gui()
+    app, win, curves, plots = build_gui()
 
-    update = make_update(reader, parser, assembler, curves)
+    update = make_update(reader, parser, assembler, curves, plots)
     timer = QtCore.QTimer()
     timer.timeout.connect(update)
     timer.start(TIMER_MS)