Decoration for polar panels

doc/get_started/the_rest.qmd

@@ -84,11 +84,12 @@ DRAW bar
 
 See how we didn't have to specify the polar coordinate system in the last example because we have a mapping to radius, allowing ggsql to deduce the coordinate system automatically.
 
-If we instead map the species to angle we end up with a rose plot
+If we instead map the species to angle we end up with a rose plot. We turn off the automatic conversion to radar plots for discrete angle categories here.
 
 ```{ggsql}
 VISUALISE species AS angle, species AS fill FROM ggsql:penguins
 DRAW bar
+PROJECT TO polar SETTING radar => false
 ```
 
 Moving back to the regular pie chart, we might be interested in comparing how the species distribution varies by sex. We can do this with faceting:

doc/syntax/coord/polar.qmd

@@ -30,6 +30,10 @@ This maps `y` to radius and `x` to angle. This is useful when converting from a
   - `0` = full pie (no hole)
   - `0.3` = donut with 30% hole
   - `0.5` = donut with 50% hole
+* `radar`: Should the plot be displayed as a radar plot? Defaults to `null`.
+  - `null` = Detect whether there is discrete data at the angle scale with more than 2 categories, in which case this becomes `true`, otherwise `false`.
+  - `true` = Display discrete data as a radar plot. This is incompatible with continuous data at the angle scale.
+  - `false` = Display classic, circular polar coordinates.
 
 ## Examples
 
@@ -70,10 +74,12 @@ This creates a gauge chart spanning from the 9 o'clock to 3 o'clock position (a
 ```{ggsql}
 VISUALISE species AS fill FROM ggsql:penguins
 DRAW bar
+SCALE angle SETTING expand => 0
 PROJECT TO polar 
   SETTING end => 270
 ```
 This creates a pie chart using only 270° (three-quarters of a circle), starting from 0° (12 o'clock) and ending at 270° (9 o'clock).
+We have turned off the scale expansion by using `SCALE angle SETTING expand => 0`.
 
 ### Donut chart with 50% hole
 ```{ggsql}
@@ -97,7 +103,23 @@ This creates a donut chart with a smaller hole (30% of the radius).
 ```{ggsql}
 VISUALISE species AS fill FROM ggsql:penguins
 DRAW bar
+SCALE angle SETTING expand => 0
 PROJECT TO polar 
   SETTING start => -90, end => 90, inner => 0.5
 ```
 This combines the `start`, `end`, and `inner` settings to create a half-circle donut chart (gauge style) spanning from 9 o'clock to 3 o'clock with a 50% hole.
+
+### Radar chart
+```{ggsql}
+WITH data(angle, radius) AS (VALUES
+  ('A', 5),
+  ('B', 2),
+  ('C', 4),
+  ('D', 7),
+  ('E', 6)
+)
+VISUALISE angle, radius FROM data
+DRAW polygon
+```
+The key to drawing a radar chart is having discrete data for the `angle` aesthetic. 
+You can turn off the radar chart by using `PROJECT TO polar SETTING radar => false`.

src/execute/mod.rs

@@ -27,6 +27,7 @@ use crate::parser;
 use crate::plot::aesthetic::{is_position_aesthetic, AestheticContext};
 use crate::plot::facet::{resolve_properties as resolve_facet_properties, FacetDataContext};
 use crate::plot::layer::is_transposed;
+use crate::plot::projection::resolve_projection_properties;
 use crate::plot::{AestheticValue, Layer, Scale, ScaleTypeKind, Schema};
 use crate::{DataFrame, DataSource, GgsqlError, Plot, Result};
 use std::collections::{HashMap, HashSet};
@@ -1413,6 +1414,13 @@ pub fn prepare_data_with_reader(query: &str, reader: &dyn Reader) -> Result<Prep
         scale::resolve_scales(spec, &mut data_map)?;
     }
 
+    // Resolve projection properties that depend on scale types (e.g., radar)
+    for spec in &mut specs {
+        if let Some(ref mut project) = spec.project {
+            resolve_projection_properties(project, &spec.scales)?;
+        }
+    }
+
     // Resolve facet properties (after data is available)
     for spec in &mut specs {
         // Get position aesthetic names from the aesthetic context (coord-specific)

src/plot/facet/types.rs

@@ -64,6 +64,23 @@ impl Facet {
     pub fn is_grid(&self) -> bool {
         self.layout.is_grid()
     }
+
+    /// Whether the given position aesthetic has free (independent) scales.
+    ///
+    /// Accepts internal position names and their variants:
+    /// `"pos1"`, `"pos1min"`, `"pos1end"`, `"pos2"`, `"pos2max"`, `"pos3"`, etc.
+    pub fn is_free(&self, aesthetic: &str) -> bool {
+        use crate::plot::ArrayElement;
+        let Some(ParameterValue::Array(arr)) = self.properties.get("free") else {
+            return false;
+        };
+        for (idx, prefix) in ["pos1", "pos2", "pos3"].iter().enumerate() {
+            if aesthetic.starts_with(prefix) {
+                return matches!(arr.get(idx), Some(ArrayElement::Boolean(true)));
+            }
+        }
+        false
+    }
 }
 
 impl FacetLayout {
@@ -221,3 +238,46 @@ impl FacetLayout {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::plot::ArrayElement;
+
+    fn facet_with_free(free: Vec<bool>) -> Facet {
+        let mut f = Facet::new(FacetLayout::Wrap {
+            variables: vec!["g".to_string()],
+        });
+        f.properties.insert(
+            "free".to_string(),
+            ParameterValue::Array(free.into_iter().map(ArrayElement::Boolean).collect()),
+        );
+        f
+    }
+
+    #[test]
+    fn is_free_checks_position_and_variants() {
+        let f = facet_with_free(vec![true, false]);
+        assert!(f.is_free("pos1"));
+        assert!(f.is_free("pos1min"));
+        assert!(f.is_free("pos1end"));
+        assert!(!f.is_free("pos2"));
+        assert!(!f.is_free("pos2max"));
+    }
+
+    #[test]
+    fn is_free_returns_false_for_material_aesthetics() {
+        let f = facet_with_free(vec![true, true]);
+        assert!(!f.is_free("color"));
+        assert!(!f.is_free("fill"));
+    }
+
+    #[test]
+    fn is_free_returns_false_without_free_property() {
+        let f = Facet::new(FacetLayout::Wrap {
+            variables: vec!["g".to_string()],
+        });
+        assert!(!f.is_free("pos1"));
+        assert!(!f.is_free("pos2"));
+    }
+}

src/plot/main.rs

@@ -739,14 +739,10 @@ mod tests {
     #[test]
     fn test_label_transform_with_default_project() {
         // LABEL x/y with default cartesian should transform to pos1/pos2
-        use crate::plot::projection::{Coord, Projection};
+        use crate::plot::projection::Projection;
 
         let mut spec = Plot::new();
-        spec.project = Some(Projection {
-            coord: Coord::cartesian(),
-            aesthetics: vec!["x".to_string(), "y".to_string()],
-            properties: HashMap::new(),
-        });
+        spec.project = Some(Projection::cartesian());
         spec.labels = Some(Labels {
             labels: HashMap::from([
                 ("x".to_string(), Some("X Axis".to_string())),
@@ -827,14 +823,10 @@ mod tests {
     #[test]
     fn test_label_transform_preserves_material() {
         // LABEL title/color should be preserved unchanged
-        use crate::plot::projection::{Coord, Projection};
+        use crate::plot::projection::Projection;
 
         let mut spec = Plot::new();
-        spec.project = Some(Projection {
-            coord: Coord::cartesian(),
-            aesthetics: vec!["x".to_string(), "y".to_string()],
-            properties: HashMap::new(),
-        });
+        spec.project = Some(Projection::cartesian());
         spec.labels = Some(Labels {
             labels: HashMap::from([
                 ("title".to_string(), Some("My Chart".to_string())),

src/plot/projection/coord/polar.rs

@@ -42,6 +42,11 @@ impl CoordTrait for Polar {
                 default: DefaultParamValue::Null,
                 constraint: ParamConstraint::number_range(0.0, 1.0),
             },
+            ParamDefinition {
+                name: "radar",
+                default: DefaultParamValue::Null,
+                constraint: ParamConstraint::boolean(),
+            },
         ];
         PARAMS
     }
@@ -75,7 +80,8 @@ mod tests {
         assert!(names.contains(&"start"));
         assert!(names.contains(&"end"));
         assert!(names.contains(&"inner"));
-        assert_eq!(defaults.len(), 4);
+        assert!(names.contains(&"radar"));
+        assert_eq!(defaults.len(), 5);
     }
 
     #[test]

src/plot/projection/mod.rs

@@ -7,5 +7,5 @@ mod resolve;
 mod types;
 
 pub use coord::{Coord, CoordKind, CoordTrait};
-pub use resolve::resolve_coord;
+pub use resolve::{resolve_coord, resolve_projection_properties};
 pub use types::Projection;