diff --git a/rfg_adc_plotter/gui/pyqtgraph_backend.py b/rfg_adc_plotter/gui/pyqtgraph_backend.py
index e56b621..4ded1d2 100644
--- a/rfg_adc_plotter/gui/pyqtgraph_backend.py
+++ b/rfg_adc_plotter/gui/pyqtgraph_backend.py
@@ -1691,9 +1691,15 @@ def run_pyqtgraph(args) -> None:
             )
             set_x_range_if_changed("line_x", p_line, f_min, f_max, padding=0)
 
-        distance_bounds = resolve_axis_bounds(runtime.ring.distance_axis)
-        if distance_bounds is not None:
-            display_axis_full = display_distance_axis_for_mode(runtime.ring.distance_axis, fft_mode)
+        # Use the SAME stable full-scale axis as the B-scan update block so both
+        # write identical bounds to the shared fft_waterfall cache. Using the
+        # per-frame runtime.ring.distance_axis here made the two thrash the cache
+        # and the B-scan y-axis jumped every frame.
+        bscan_axis = stable_full_scale_distance_axis()
+        if bscan_axis is None:
+            bscan_axis = runtime.ring.distance_axis
+        if bscan_axis is not None:
+            display_axis_full = display_distance_axis_for_mode(bscan_axis, fft_mode)
             display_bounds = resolve_axis_bounds(display_axis_full)
             if display_bounds is not None:
                 d_min_display, d_max_display = display_bounds