docs: horizontal plot split

docs/USER-GUIDE.md

@@ -71,6 +71,29 @@ it (type, size, range, sample rate, timing). Below come the colour and the **Plo
 rows. Gain and offset are applied at ingest, so the measurements and the export carry the same numbers the
 plot shows; a renamed unit is used there too and is saved with the session.
 
+### Splitting a row
+
+Press `l` (or pick **Layout editor** from the overflow menu) to arrange the plots: a **`×`** at the top
+left of each plot removes it, a **`+`** on each boundary between rows adds a row, and a **Done** button
+in the plot's top-right corner leaves the editor. The plots can also be put **side by side**: a row holds
+up to **4** plots, and the figure up to **8** in all.
+
+- **To split a row**, click the `+` at its left or right edge, or the `+` between two of its plots. The
+  new plot is empty and the plots after it keep their traces. The `+` is dim at 4 plots in the row or 8
+  in the figure.
+- **To resize**, drag the arrow between two plots left or right. The two neighbours trade width (each
+  stays at least about 96 px wide); the sizes are saved with the session.
+- **To remove a plot**, click its `×`. In a split row it removes just that plot and its traces; in a row
+  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
+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
+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.
+
 ## 4. The window and navigation
 
 The header's window control (`2 s`) sets the time span on screen; the popover takes a number and a
@@ -129,8 +152,8 @@ 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;
-- **horizontal** — two rulers per row;
+- **vertical** — two times, shared by every row (and every plot of a split row);
+- **horizontal** — two rulers per plot;
 - **both** — the two together;
 - **mix** — the two times, read off each *trace* where they cross it.