docs: split rows share one y axis

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

help/C/org.udpscope.app/layout.page

@@ -57,7 +57,9 @@
     </steps>
     <p>All the plots of a row, and of the whole figure, share one time window: zooming or
     panning along time in any of them moves all of them, and the vertical cursors are drawn in
-    every plot. The y axes, the horizontal cursors and the fits belong to each plot. Only the
+    every plot. The plots of a split row also share one y range: the y labels are drawn at the
+    left of the first plot only, the horizontal cursors sit at the same values in every plot, and a
+    zoom, pan, fit or fixed range in any of them applies to the whole row. Only the
     bottom row carries the time labels. A row holds at most 4 plots and the figure at most 8
     in total; the <gui>+</gui> buttons go grey at the limit.</p>
   </section>
@@ -66,7 +68,7 @@
     <title>Worth knowing</title>
     <list>
       <item><p>A plot's own <gui>right-click menu</gui> (the same compact rows as the app's
-      menu) holds the y-axis scale, the two fits, and <gui>Remove signal</gui>, which swaps
+      menu) holds the y-axis scale (for the whole row), the two fits, and <gui>Remove signal</gui>, which swaps
       the menu to that plot's traces plus <gui>All traces on this plot</gui> — with a
       <gui>Back</gui> row to return. That is how a trace comes off a row without going through
       the layout editor.</p></item>