docs: split rows share one y axis

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

README.md

@@ -2580,7 +2580,8 @@ the only place it is visible. Outside a gesture the plot is not read at all: `cu
 snapped value then, which is what the app wrote itself a frame earlier, and adopting that back
 is what made a dragged cursor spring to where it had been. A ruler is per row: each row has its own y axis, so two rows' rulers are
 two different values, and the pane names the row. In a split row (see "Splitting a row") the vertical pair is drawn in
-every plot and a ruler belongs to one plot, which the pane names `plot 2a`.
+every plot, and so is the horizontal pair: the plots of a split row share one y axis, so the pane lists one ruler
+line for the whole row, and dragging a ruler in any plot moves it in all of them.
 
 `mix` needs no rulers: it reports the value the trace actually has at each marker, read
 through the library's `Plot::crosshair_value()` — **the nearer of the two bracketing
@@ -2826,11 +2827,15 @@ reach every position the two-per-row reading offers.
 A row can be split into **two to four plots side by side**, up to **eight plots in all**. A split is
 only an arrangement of more plots on the screen: **every plot shares the one time window** -- one x
 link group, so a pan or zoom in any plot moves them all; one crosshair, drawn at the same time in every
-plot; and one pair of vertical cursors, at the same two times in every plot. What each plot keeps is its
-own **y axis** (auto or fixed), its own horizontal rulers and its own fit; a wheel or an axis drag acts on
-the plot under the pointer. Only the **bottom** row's plots carry the time labels, and the left margin
+plot; and one pair of vertical cursors, at the same two times in every plot. The plots of a row
+also share **one y range**, the way the rows share one x window: the row has one y axis (auto or fixed),
+drawn with its labels at the left of the first plot only, one pair of horizontal rulers drawn at the same
+value in every plot, and one fit. A wheel, a drag, a box zoom, **Fit all** and the Auto/Fixed dialog act on
+the whole row whichever plot they land in, and an auto-scaled row fits the union of its plots' traces (a plot
+with none is ignored). Only the **bottom** row's plots carry the time labels, and the left margin
 holding the y labels is one width for the whole figure, so the plots of a row line up with the rows
-above and below it; a plot whose labels are narrower simply has more margin.
+above and below it; a plot whose labels are narrower simply has more margin. Because only the first plot
+has a margin, the plots of a row get equal plot areas at equal weights.
 
 In the **Layout editor** (`l`):
 
@@ -2847,8 +2852,8 @@ In the **Layout editor** (`l`):
 
 Plots are named by their row and, in a split row, a letter: **`1`**, **`2a`**..**`2d`**. The names are
 what the signals page's *Plot on* buttons (tooltip "row 2, cell 1 of 2"), the status notices ("plotting
-'sine' on plot 2b") and the cursors pane for a split row (`plot 2a`; a plot of an unsplit row stays
-`row N`, counted from 0 as before) show. A signal dragged from its badge lands on the plot under the
+'sine' on plot 2b") and the cursors pane (one ruler line per figure row, `row 2` counted from 1 for a split row; a plot of an
+unsplit row stays `row N`, counted from 0 as before) show. A signal dragged from its badge lands on the plot under the
 pointer, with the same free-slot rule as a row. The session file keeps the arrangement (`row` and `width`
 on each `[[plot]]`, see "The session layout file").