plot host: rows hold 1..4 cells; split probe scaffold

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

src/plot_host.hpp

@@ -155,8 +155,43 @@ class RowSet {
     double ruler_b_y = 0.0;
   };
 
+  /// The number of **plots** (cells over all figure rows), which is what `row(i)` indexes. With no
+  /// row split it is also the number of figure rows; `figure_row_count()` is the latter.
   int row_count() const noexcept { return rows_; }
 
+  /// A figure row holds one to `kMaxCellsPerRow` plots side by side, sharing the row's height and
+  /// the figure's one x window. The flattened plot index (`row(i)`, `Row::plot`, the figure's
+  /// `plot(i)`) runs row-major: row 0's cells left to right, then row 1's.
+  static constexpr int kMaxCellsPerRow = 4;
+  int figure_row_count() const noexcept { return static_cast<int>(cells_per_row_.size()); }
+  /// How many cells figure row `fr` holds; 0 for a row that does not exist.
+  int cells_in_row(int fr) const;
+  /// The flattened index of figure row `fr`'s first cell; `row_count()` for `fr ==
+  /// figure_row_count()`.
+  int first_plot_of_row(int fr) const;
+  /// The figure row plot `plot` is in, or -1 for an index that is not a plot.
+  int figure_row_of(int plot) const;
+  /// Plot `plot`'s 0-based position inside its row, or -1.
+  int cell_of(int plot) const;
+  /// Figure row `fr`'s cell width shares (empty means equal; always empty for a single-cell row).
+  const std::vector<float>& cell_weights(int fr) const;
+  /// What the UI calls a plot: the figure row's number (`"2"`) for a single-cell row, the number
+  /// and a letter (`"2a"`..`"2d"`) for a split one.
+  std::string plot_label(int plot) const;
+
+  /// Adds an empty cell to figure row `fr` at position `at` (clamped to the row), pushing the
+  /// cells at and after it right. False at `kMaxCellsPerRow` cells or `kMaxRows` plots.
+  bool insert_cell(int fr, int at);
+  /// Removes plot `plot`. A single-cell row removes the row itself (`remove_row()`); false when
+  /// that would leave no plot.
+  bool remove_cell(int plot);
+  /// Figure row `fr`'s cell width shares, normalised by the plotter; empty means equal.
+  void set_cell_weights(int fr, std::vector<float> w);
+  /// Replaces the whole arrangement: `cells[r]` plots in figure row `r`, each clamped to
+  /// `1..kMaxCellsPerRow` and the total to `kMaxRows`. Weights are reset; plot contents stay in
+  /// their flattened slots.
+  void set_cells_per_row(std::vector<int> cells);
+
   /// The alpha every trace is drawn with. The *group* views use it: accumulating events wants their
   /// traces translucent so that agreement between them reads as brightness, which is the whole point
   /// of a persistence display. It is mixed into the palette colour where the trace is created, so it
@@ -169,22 +204,24 @@ class RowSet {
   /// trace set or the row count changes, so weights that lived only in the figure would be lost on the
   /// first drop or row change after a user had sized their layout.
   void set_row_weights(std::vector<float> weights);
-  /// Sizes the stored weights to the rows that exist and hands them to the figure.
+  /// Sizes the stored weights to the figure rows that exist and hands them to the figure. Weights
+  /// are per **figure row**, not per plot.
   void apply_row_weights();
   const std::vector<float>& row_weights() const noexcept { return row_weights_; }
-  /// Shows `rows` rows. A changed count rebuilds the figure, since a `Figure`'s row
-  /// count is fixed at construction.
+  /// Shows `rows` figure rows of one cell each (so `rows` plots, any split undone). A changed
+  /// arrangement rebuilds the figure, since a `Figure`'s shape is fixed at construction.
   void set_row_count(int rows);
-  /// Inserts an empty row at `at` (0 = above the first row, `row_count()` = below the
-  /// last), pushing the rows at and below it down. Returns false at `kMaxRows`.
+  /// Inserts an empty single-cell figure row at `at` (0 = above the first row,
+  /// `figure_row_count()` = below the last), pushing the rows at and below it down with their
+  /// cells. Returns false at `kMaxRows` plots.
   ///
   /// The rows below move as *rows*: their trace assignments, colours and cursors follow
   /// them, so inserting above row 1 does not renumber what row 1 was plotting. The
   /// figure's `Plot`s are not what moves -- they are rebuilt and re-dealt -- which is
   /// the same mechanism a row-count change already used.
   bool insert_row(int at);
-  /// Removes row `at` and pulls the rows below it up. Returns false when only one row
-  /// is left: a figure with no rows has nothing to draw and no way back.
+  /// Removes figure row `at` (with all its cells) and pulls the rows below it up. Returns false
+  /// when only one row is left: a figure with no rows has nothing to draw and no way back.
   bool remove_row(int at);
   Row& row(int i) { return storage_[static_cast<std::size_t>(i)]; }
   const Row& row(int i) const { return storage_[static_cast<std::size_t>(i)]; }
@@ -563,9 +600,15 @@ class RowSet {
   std::unique_ptr<plot::Figure> figure_;
   /// The one x link group every visible row belongs to. Groups are numbered from 1.
   plot::u32 x_group_ = 0;
+  /// The number of plots; `sum(cells_per_row_) == rows_` always.
   int rows_ = 1;
+  /// Cells per figure row, and each row's cell width shares (empty = equal; kept empty for a
+  /// single-cell row). Both are state of the `RowSet`, restated to every new figure.
+  std::vector<int> cells_per_row_{1};
+  std::vector<std::vector<float>> cell_weights_ = std::vector<std::vector<float>>(1);
+  void apply_cell_weights();
   float trace_alpha_ = 1.0f;
-  std::vector<float> row_weights_;
+  std::vector<float> row_weights_;  ///< per figure row
   double window_s_ = 2.0;
   double trigger_marker_x_ = 0.0;
   bool trigger_marker_on_ = false;