layout editor: horizontal-resize icon, divider drag follows the pointer; x caption on every labelled plot; probes and docs

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

README.md

@@ -2832,10 +2832,13 @@ also share **one y range**, the way the rows share one x window: the row has one
 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
+with none is ignored). The **bottom** row's plots carry the time labels, and so does any row above it whose
+columns differ from the row below's (the `t0 = ...` caption goes with the labels, on each plot that draws
+them); a row whose columns match the next one's leaves its labels to it. 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. Because only the first plot
-has a margin, the plots of a row get equal plot areas at equal weights.
+above and below it; a plot whose labels are narrower simply has more margin. Only the first plot has a left
+margin and only the last a right one, so the plots of a row get equal plot areas at equal weights and
+neighbouring plots are exactly one column gap apart.
 
 In the **Layout editor** (`l`):
 
@@ -2843,7 +2846,7 @@ In the **Layout editor** (`l`):
   empty plot is added at that position and the plots after it move right *with their traces*. A one-cell
   row can be split from either side. The `+` is dim at 4 plots in the row or 8 plots in all (the row `+`,
   which adds a whole row, dims at 8 plots).
-- A **drag arrow between two plots** resizes the two neighbours against each other; neither can be
+- A **double-headed (left-right) arrow between two plots** resizes the two neighbours against each other; neither can be
   dragged below about 96 px. The plotter keeps 64 px as an absolute floor, and when the window is too
   narrow for that it shrinks every plot proportionally and drops the gap, so nothing has a negative size.
   The widths are saved when the drag ends.