feat(render): independent major and minor grid classes tied to the tick set

src/plot/grid.cpp

@@ -26,16 +26,20 @@ void Grid::build(const std::vector<Tick>& x_ticks, Range x_range,
 
   // Segments stay in data space: the renderer applies the transform, so a pan or
   // zoom needs no grid rebuild other than for a changed tick set.
+  //
+  // Both classes are built: the tick set is the only source of positions, and
+  // `Style::grid_*_visible` decides what the renderer stages from here, so a class
+  // is never filtered out of the tick set (that would be a second, geometry-side
+  // switch for the same behaviour, and the grid would then disagree with the tick
+  // list a caller can read).
   if (vertical) {
     for (const Tick& tick : x_ticks) {
-      if (!tick.major && !show_minor) continue;
       if (!std::isfinite(tick.value) || !inside(tick.value, x_range)) continue;
       lines_.push_back(GridLine{tick.value, y_range.min, tick.value, y_range.max, tick.major});
     }
   }
   if (horizontal) {
     for (const Tick& tick : y_ticks) {
-      if (!tick.major && !show_minor) continue;
       if (!std::isfinite(tick.value) || !inside(tick.value, y_range)) continue;
       lines_.push_back(GridLine{x_range.min, tick.value, x_range.max, tick.value, tick.major});
     }