Correct downsampling and better layout

src/plot_host.cpp

@@ -54,6 +54,53 @@ void RowSet::set_row_count(int rows) {
   rebuild_all();
 }
 
+void RowSet::carry_row(Row& dst, const Row& src) {
+  dst.signal_name = src.signal_name;
+  dst.name = src.name;
+  dst.channel = src.channel;
+  dst.color_index = src.color_index;
+  dst.cursors_visible = src.cursors_visible;
+  dst.cursor_a_x = src.cursor_a_x;
+  dst.cursor_b_x = src.cursor_b_x;
+  dst.ruler_y = src.ruler_y;
+  // The slot's series stays where it is, but what it is bound to belonged to the row
+  // this state came *from*. `bind()` re-resolves every slot next frame; clearing it here
+  // is what stops the frame in between from drawing the moved row's data under the new
+  // row's legend.
+  for (SlotSeries& s : dst.slots) s.bind(nullptr);
+}
+
+void RowSet::clear_row(Row& r) {
+  r.signal_name = {};
+  r.name = {};
+  r.channel = {-1, -1, -1, -1};
+  r.color_index = {0, 1, 2, 3};
+  r.cursors_visible = false;
+  r.cursor_a_x = 0.0;
+  r.cursor_b_x = 0.0;
+  r.ruler_y = 0.0;
+  for (SlotSeries& s : r.slots) s.bind(nullptr);
+}
+
+bool RowSet::insert_row(int at) {
+  if (rows_ >= kMaxRows) return false;
+  const int pos = std::clamp(at, 0, rows_);
+  for (int i = rows_; i > pos; --i) carry_row(row(i), row(i - 1));
+  clear_row(row(pos));
+  ++rows_;
+  rebuild_all();
+  return true;
+}
+
+bool RowSet::remove_row(int at) {
+  if (rows_ <= 1 || at < 0 || at >= rows_) return false;
+  for (int i = at; i + 1 < rows_; ++i) carry_row(row(i), row(i + 1));
+  --rows_;
+  clear_row(row(rows_));
+  rebuild_all();
+  return true;
+}
+
 void RowSet::install(Row& r, int /*row_index*/, plot::Plot& p) {
   r.plot = &p;
   r.axis_x = r.plot->add_x_axis(