docs: split rows share one y axis

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

docs/USER-GUIDE.md

@@ -87,10 +87,12 @@ up to **4** plots, and the figure up to **8** in all.
   of one plot it removes the row.
 
 Every plot shares **one time window**: panning or zooming in any of them moves them all, the crosshair
-sits at the same time in each, and the vertical cursors are the same two times in each. Each plot has
-its **own y axis**, its own horizontal rulers and its own fit, and only the bottom row shows the time
+sits at the same time in each, and the vertical cursors are the same two times in each. 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 rulers are the same two values in every plot, and a y zoom, pan, fit or fixed range in any of
+them applies to the whole row. Only the bottom row shows the time
 labels. A plot of a split row is named by its row and a letter, `2a` to `2d`: that is what the signals
-page's **Plot on** buttons, the status line and the cursors readout call it, and a signal dragged
+page's **Plot on** buttons and the status line call it, and a signal dragged
 from its badge goes onto the plot under the pointer. The arrangement is saved per source, and a session
 saved before rows could be split opens with one plot per row, as it was.
 
@@ -153,7 +155,7 @@ does not walk it off its snapshot.
 The cursors toggle in the header bar (`c`) switches on two instruments per row:
 
 - **vertical** — two times, shared by every row (and every plot of a split row);
-- **horizontal** — two rulers per plot;
+- **horizontal** — two rulers per row (every plot of a split row shows the same two values);
 - **both** — the two together;
 - **mix** — the two times, read off each *trace* where they cross it.