Correct downsampling and better layout

src/plot_host.hpp

@@ -41,7 +41,12 @@ namespace udpscope {
 /// context and is simply handed the new figure.
 class RowSet {
  public:
-  static constexpr int kMaxRows = 4;
+  /// The ceiling on rows, not the count: rows are added and removed one at a time
+  /// (`insert_row()`, `remove_row()`) rather than picked off a list of fixed layouts.
+  /// A cap still exists because `storage_` is the rows' persistent state and has to be
+  /// allocated somewhere, and because the figure keeps a floor height per row -- past
+  /// about this many a row is chrome and no plot.
+  static constexpr int kMaxRows = 8;
   static constexpr int kSlotsPerRow = 4;
 
   RowSet();
@@ -93,6 +98,17 @@ class RowSet {
   /// Shows `rows` rows. A changed count rebuilds the figure, since a `Figure`'s row
   /// count 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`.
+  ///
+  /// 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.
+  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)]; }
 
@@ -192,6 +208,15 @@ class RowSet {
   static plot::Color unplotted_color();
 
  private:
+  /// Moves one row's *persistent* state -- what it plots, in which colours, where its
+  /// cursors are -- onto another row. Deliberately not a `Row` assignment: a `Row` also
+  /// carries the figure handles (`plot`, the axis and cursor ids) and the `SlotSeries`
+  /// objects the traces point at, and those belong to the row's *position*, not to its
+  /// contents. `rebuild_all()` re-deals them straight after.
+  static void carry_row(Row& dst, const Row& src);
+  /// Back to an empty row: nothing plotted, the default palette entries, no cursors.
+  static void clear_row(Row& r);
+
   void install(Row& r, int row_index, plot::Plot& p);
   void place_cursors(Row& r);
   /// The newest x any row's live traces hold, which is what the group follows.