plot host: split rows share y, rulers per row

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

src/ui.cpp

@@ -1752,7 +1752,8 @@ GtkWidget* cursors_header(App& app) {
 
 /// The unit a row's y rulers are in: the one its bound channels agree on, or none when they
 /// disagree -- a ruler sits on the row's y axis, and two signals of different units on one
-/// axis make "the" unit a guess.
+/// axis make "the" unit a guess. The cells of a split row share that axis, so the channels of
+/// every cell of `row`'s figure row count.
 ///
 /// The channel list is rebuilt by the producer, so it is read under the model mutex, like
 /// every other read of it from this thread.
@@ -1760,18 +1761,23 @@ std::string row_unit(App& app, int row) {
   std::lock_guard<std::mutex> lock(app.model->mutex());
   std::string unit;
   bool first = true;
-  for (int s = 0; s < RowSet::kSlotsPerRow; ++s) {
-    const std::size_t si = static_cast<std::size_t>(s);
-    const RowSet::Row& r = app.rows->row(row);
-    if (r.signal_name[si].empty()) continue;
-    const int ch = r.channel[si];
-    if (ch < 0 || static_cast<std::size_t>(ch) >= app.model->channels().size()) continue;
-    const std::string& u = app.model->channels()[static_cast<std::size_t>(ch)].unit;
-    if (first) {
-      unit = u;
-      first = false;
-    } else if (unit != u) {
-      return "";
+  const int fr = app.rows->figure_row_of(row);
+  const int begin = fr >= 0 ? app.rows->first_plot_of_row(fr) : row;
+  const int end = fr >= 0 ? begin + app.rows->cells_in_row(fr) : row + 1;
+  for (int p = begin; p < end; ++p) {
+    const RowSet::Row& r = app.rows->row(p);
+    for (int s = 0; s < RowSet::kSlotsPerRow; ++s) {
+      const std::size_t si = static_cast<std::size_t>(s);
+      if (r.signal_name[si].empty()) continue;
+      const int ch = r.channel[si];
+      if (ch < 0 || static_cast<std::size_t>(ch) >= app.model->channels().size()) continue;
+      const std::string& u = app.model->channels()[static_cast<std::size_t>(ch)].unit;
+      if (first) {
+        unit = u;
+        first = false;
+      } else if (unit != u) {
+        return "";
+      }
     }
   }
   return unit;
@@ -1840,10 +1846,14 @@ void rebuild_cursors_pane(App& app) {
     for (int r = 0; r < app.rows->row_count(); ++r) {
       if (!rulers) break;
       char who[64];
-      // A row split into cells names its plots the way the rest of the UI does ("plot 2a"); an
+      const int fr = app.rows->figure_row_of(r);
+      const bool split = app.rows->cells_in_row(fr) > 1;
+      // A ruler pair belongs to the figure row (its cells share one y axis), so a split row gets one
+      // line, on its first cell, named by the row's number -- the one `plot_label()` leads with. An
       // unsplit row keeps the wording it always had.
-      if (app.rows->cells_in_row(app.rows->figure_row_of(r)) > 1) {
-        std::snprintf(who, sizeof(who), "plot %s", app.rows->plot_label(r).c_str());
+      if (split && app.rows->cell_of(r) != 0) continue;
+      if (split) {
+        std::snprintf(who, sizeof(who), "row %d", fr + 1);
       } else {
         std::snprintf(who, sizeof(who), "row %d", r);
       }