plot host: split rows share y, rulers per row

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

docs/claude/probe-readings.md

@@ -2857,7 +2857,8 @@ inside the panel does not close it), `click_outside_closes=1` (a click on the pl
 [split-probe] weights 3:1    plots=2 frows=1 cells=[2] widths=[533,220] tile=1 x_equal=1 cursors_aligned=1 weights=[3,1] traces=[1a:sine|1b:square]
 [split-probe] caps-calls      insert_cell=[1,1,0] refused_cell=1 insert_row=[1,1,0] refused_row=1 plots=8
 [split-probe] roundtrip       rt=1 expect=r0(h=2.0000)[w=1.0000:sine] r1(h=1.0000)[w=3.0000:square][w=1.0000:ramp] got=r0(h=2.0000)[w=1.0000:sine] r1(h=1.0000)[w=3.0000:square][w=1.0000:ramp]
-[split-probe] ruler-labels    split=[plot 1a,plot 1b,plot 2a,plot 2b] mixed=[row 0,plot 2a,plot 2b]
+[split-probe] ruler-labels    split=[row 1,row 2] mixed=[row 0,row 2]
+[split-probe] shared-y        y_shared=1 y_gutter=1 y_fit=1 ruler_mirror=1 ranges=[A=-3.000000..7.000000 B=-3.000000..7.000000 | fit A=-1.04..1.04 B=-1.04..1.04 data=-1..1] axes_w=[642.4,642.4] auto=1
 ```
 
 `UDPSCOPE_PROBE_SPLIT_MS` (stderr; see [verification.md](verification.md)). Nineteen steps, 250 ms apart;
@@ -2903,7 +2904,8 @@ The other lines, in order:
 | `plot-on-buttons` | `[1a:row 1, cell 1 of 2\|1b:row 1, cell 2 of 2\|2a:row 2, cell 1 of 2\|2b:row 2, cell 2 of 2]` | a signals-page button labelled with the flat index, or without the cell tooltip; a one-cell row keeps its plain label and the tooltip "draw this signal on this plot" |
 | `assign-1b` / `drop-2b` | `assigned=1 notice='plotting 'sine' on plot 1b'`; `dropped=1 notice='plotting 'square' on plot 2b'` | a notice that names the plot by number alone |
 | `after-assign` (state line) | `traces=[1a:-\|1b:sine\|2a:-\|2b:square]` | the signal went to a neighbouring cell |
-| `ruler-labels` | `split=[plot 1a,plot 1b,plot 2a,plot 2b]` and, with cells `{1,2}`, `mixed=[row 0,plot 2a,plot 2b]` | the cursors pane's per-plot ruler line (a label read through the pane's own widgets): a split row's plots are `plot <label>`, an unsplit row keeps `row <plot index>` (**0-based**, as it always was -- `row 0` is the first plot, not a defect) |
+| `ruler-labels` | `split=[row 1,row 2]` and, with cells `{1,2}`, `mixed=[row 0,row 2]` | the cursors pane's ruler lines (a label read through the pane's own widgets): **one line per figure row**, because the cells of a split row share their y axis and so their rulers. A split row is `row <figure row, from 1>` and sits on its first cell; an unsplit row keeps `row <plot index>` (**0-based**, as it always was -- `row 0` is the first plot, not a defect). One line per cell (`plot 1a,plot 1b,...`) is the pre-sharing reading |
+| `shared-y` | `y_shared=1 y_gutter=1 y_fit=1 ruler_mirror=1`, `ranges=[A=-3..7 B=-3..7 | fit A=B]`, `axes_w` equal to within a pixel, `auto=1` | a row of two cells with a trace each. `y_shared`: a `set_y_range(-3, 7)` on cell A is B's after one frame. `y_gutter`: only cell A has a y gutter (B's `y_axis_width` and `y_label_area.w` are 0) and the two plots are equally wide. `y_fit`: `fit_all()` leaves both cells on one range that covers both traces (not the manual one). `ruler_mirror`: a ruler written on A and read back by `sync_cursors(0)` is B's too. A 0 in the first two is `set_share_row_y(true)` not reaching the figure; in `y_fit`, no frame between `fit_all()` and the read (the probe restructures to pump one) or a group that did not follow the flag |
 
 The run ends with the usual `gtk warnings: 0` and no `Gtk-CRITICAL`. `x_equal=1` and `cursors_aligned=1` on every state line are
 the only readings of the shared time window across cells.

docs/claude/verification.md

@@ -205,15 +205,15 @@ precedent, not an exception.
   with, so a synchronous sequence would read every position one action stale. It clicks by
   emitting `::clicked`, never `gtk_widget_activate()`, which holds a `GtkButton` for its
   250 ms `ACTIVATE_TIMEOUT` and then emits into a button the rebuild has already destroyed.
-- `UDPSCOPE_PROBE_SPLIT_MS` — drive the horizontal plot split, 19 steps 250 ms apart (so a run needs
-  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=7500` at least; it enables and assigns the
+- `UDPSCOPE_PROBE_SPLIT_MS` — drive the horizontal plot split, 23 steps 250 ms apart (so a run needs
+  `UDPSCOPE_PROBE_SPLIT_MS=1500 UDPSCOPE_EXIT_AFTER_MS=9500` at least; it enables and assigns the
   signals itself, so `UDPSCOPE_ENABLE_ALL_MS` is not needed). Each step restructures the cells,
   re-points the area at the new figure and lets the *next* step read a frame that was actually laid out
   with them, which is why every report line describes the previous step's change. It splits and
   weights a row, fills the caps, round-trips a session through `collect_layout()` / `save_layout()` /
   `load_layout()` / `apply_layout()` (and a copy with its `row`/`width` lines stripped), clicks the layout
   editor's real cell `+` button, drives the column drag handler, and checks the signals page, the drop's
-  `row_at()` and the cursors pane's ruler labels. Run it once, serially, on its own mock port, from a
+  `row_at()`, the cursors pane's ruler labels and the shared y axis of a split row (`shared-y`). Run it once, serially, on its own mock port, from a
   fresh `XDG_CONFIG_HOME` (the probe deletes the temp files `udpscope-split-probe.toml` and `.legacy` it
   writes under `g_get_tmp_dir()`, but the editor's clicks schedule a real session save). Read it against
   the `[split-probe]` section of [probe-readings.md](probe-readings.md); the run must still end with