[#19]:svarga:feature, add plot::donut driver with adjustable plot::hole

doxy/docs/index.md

@@ -132,7 +132,7 @@ shown above.
 - `plot::ohlc(t, open, high, low, close, opts...)` — open/high/low/close candles *(coming in the gallery)*.
 - `plot::graph(nodes, edges, opts...)` — node/edge network layout *(coming in the gallery)*.
 - `plot::pie(labels, values, opts...)` — proportional wedges *(coming in the gallery)*.
-- `plot::pie(labels, values, plot::donut{0.55}, opts...)` — donut (pie with an inner-radius hole).
+- `plot::donut(labels, values, opts...)` — donut (pie with a default inner-radius hole; adjust with `plot::hole{r}`).
 - `plot::hexbin(xs, ys, opts...)` — hexagonal density binning *(coming in the gallery)*.
 
 ## Gallery

examples/pie.cpp

@@ -11,7 +11,7 @@ int main(){
 	plot::save("pie.svg",
 		plot::pie(labels, share, plot::title("language share"), plot::width{520}, plot::height{520}));
 
-	plot::save("donut.svg", plot::pie(labels, share, plot::donut{0.55},
+	plot::save("donut.svg", plot::donut(labels, share,
 		plot::title("language share (donut)"), plot::width{520}, plot::height{520}));
 	return 0;
 }

include/plot/pie.hpp

@@ -1,10 +1,11 @@
 /* Copyright (c) 2026 Steven Varga, Toronto, ON, Canada
  * MIT License — see LICENSE
  *
- * plot::pie(labels, values, opts...) — a deferred view (plot::view) drawing a
- * pie chart, or a donut when plot::donut{r} (inner-radius fraction in (0,1)) is
- * given. Each slice is a FILLED <polygon> wedge: the arc is segmented into short
- * chords, centre→arc→centre for a pie, or outer-arc→inner-arc for a donut ring.
+ * plot::pie(labels, values, opts...) and plot::donut(labels, values, opts...) —
+ * deferred views (plot::view) drawing a pie / donut. donut defaults to a 0.55
+ * inner-radius hole; adjust either with plot::hole{r} (r in (0,1)). Each slice is
+ * a FILLED <polygon> wedge: the arc is segmented into short chords, centre→arc→
+ * centre for a pie, or outer-arc→inner-arc for a donut ring.
  * Slices are filled with the theme colour. Slice colours cycle theme.series; a
  * label is placed at the slice's mid-angle.
  *
@@ -31,21 +32,22 @@
 #include "view.hpp"
 
 namespace plot {
-	// donut inner-radius fraction in (0,1); 0 (or absent) ⇒ a full pie.
-	struct donut { using value_type = tag::donut_t; double value; };
+	// inner-radius hole fraction in (0,1); 0 (or absent) ⇒ a full pie. Set by
+	// plot::donut (default 0.55) and accepted by plot::pie too.
+	struct hole { using value_type = tag::hole_t; double value; };
 }
 
 namespace plot::impl {
 	template <class... opt_t>
-	double donut_of(const opt_t&... opts){
-		using d_t = typename arg::tpos<tag::donut_t, opt_t...>;
+	double hole_of(double fallback, const opt_t&... opts){
+		using d_t = typename arg::tpos<tag::hole_t, opt_t...>;
 		if constexpr( d_t::present ){
 			auto tuple = std::forward_as_tuple(opts...);
 			double r = std::get<d_t::position>(tuple).value;
 			if( r < 0 ) r = 0; else if( r > 0.95 ) r = 0.95;
 			return r;
 		}
-		return 0.0;
+		return fallback;
 	}
 
 	template <class... opt_t>
@@ -54,6 +56,7 @@ namespace plot::impl {
 		std::vector<std::string> labels;
 		std::vector<double> values;
 		std::tuple<opt_t...> opts;
+		double default_inner = 0.0;   // plot::pie ⇒ 0 (full); plot::donut ⇒ 0.55
 
 		std::pair<std::size_t,std::size_t> natural() const {
 			return std::apply([](const auto&... o){
@@ -63,7 +66,7 @@ namespace plot::impl {
 			std::apply([&](const auto&... o){
 				const theme_t& th = impl::resolve_theme(o...);
 				auto [title, xl, yl] = impl::texts(o...);
-				const double inner = impl::donut_of(o...);
+				const double inner = impl::hole_of(default_inner, o...);
 				using attribute_t = plot::attribute::element_t;
 
 				const std::size_t n = std::min(labels.size(), values.size());
@@ -143,7 +146,15 @@ namespace plot {
 	impl::pie_view<opt_t...> pie(std::vector<std::string> labels,
 			std::vector<double> values, opt_t... opts){
 		return impl::pie_view<opt_t...>{ std::move(labels), std::move(values),
-				std::make_tuple(opts...) };
+				std::make_tuple(opts...) };   // default_inner = 0 → full pie
+	}
+
+	// donut: a pie with a default inner-radius hole (0.55); adjust with plot::hole{}.
+	template <class... opt_t>
+	impl::pie_view<opt_t...> donut(std::vector<std::string> labels,
+			std::vector<double> values, opt_t... opts){
+		return impl::pie_view<opt_t...>{ std::move(labels), std::move(values),
+				std::make_tuple(opts...), 0.55 };
 	}
 }
 #endif

include/plot/tags.hpp

@@ -48,7 +48,7 @@ namespace plot::tag {
 	struct bins_t{};      // histogram bin count
 	struct bandwidth_t{}; // KDE bandwidth
 	struct levels_t{};    // contour iso-level count
-	struct donut_t{};     // pie inner-radius fraction (0 = full pie)
+	struct hole_t{};      // pie/donut inner-radius hole fraction (0 = full pie)
 	struct hex_t{};       // hexagonal heatmap style marker
 	struct wireframe_t{}; // outline-only bars/bins (no fill) — issue #13
 }

test/plot_fill.cpp

@@ -51,7 +51,7 @@ int main(){
 	{
 		std::vector<std::string> labels{"x","y","z","w"};
 		std::vector<double> vals{30,20,40,10};
-		auto out = render_of(plot::pie(labels, vals, plot::donut{0.5}, plot::title("donut")));
+		auto out = render_of(plot::donut(labels, vals, plot::title("donut")));
 		if(!well_formed(out)) return 4;
 		if(out.find("<polygon") == std::string::npos) return 5;
 		if(!has_filled_polygon(out)) return 6;

test/plot_new.cpp

@@ -103,9 +103,13 @@ int main(){
 		if(!well_formed(pie)) return 17;
 		if(pie.find("<polygon") == std::string::npos) return 18;  // filled wedges
 		if(pie.find(">x<")       == std::string::npos) return 19;  // slice label
-		auto dn = render_of(plot::pie(labels, vals, plot::donut{0.5}, plot::title("donut")));
+		auto dn = render_of(plot::donut(labels, vals, plot::title("donut")));
 		if(!well_formed(dn)) return 20;
 		if(dn.find("<polygon") == std::string::npos) return 21;   // filled ring wedges
+		// plot::hole{} overrides the donut's default inner radius
+		auto dh = render_of(plot::donut(labels, vals, plot::hole{0.3}, plot::title("donut")));
+		if(!well_formed(dh)) return 22;
+		if(dh.find("<polygon") == std::string::npos) return 23;
 	}
 
 	// ---- hexbin (hexagonal heatmap) -----------------------------------------