fix: wrap hue in HSLColor + add from_degrees

plotters/src/style/color.rs

@@ -139,15 +139,29 @@ impl BackendStyle for RGBColor {
 #[cfg_attr(feature = "serialization", derive(Serialize, Deserialize))]
 pub struct HSLColor(pub f64, pub f64, pub f64);
 
+impl HSLColor {
+    #[inline]
+    pub fn from_degrees(h_deg: f64, s: f64, l: f64) -> Self {
+        Self(h_deg / 360.0, s, l)
+    }
+}
+
 impl Color for HSLColor {
     #[inline(always)]
     #[allow(clippy::many_single_char_names)]
     fn to_backend_color(&self) -> BackendColor {
-        let (h, s, l) = (
-            self.0.clamp(0.0, 1.0),
-            self.1.clamp(0.0, 1.0),
-            self.2.clamp(0.0, 1.0),
-        );
+        // Hue: do not clamp; normalize
+        // - If >1.0, treat as degrees (divide by 360), then wrap to [0,1)
+        // - Else, wrap value to [0,1) to accept negatives
+        let h = if self.0 > 1.0 {
+            (self.0 / 360.0).rem_euclid(1.0)
+        } else {
+            self.0.rem_euclid(1.0)
+        };
+
+        // Saturation & lightness remain clamped to valid ranges
+        let s = self.1.clamp(0.0, 1.0);
+        let l = self.2.clamp(0.0, 1.0);
 
         if s == 0.0 {
             let value = (l * 255.0).round() as u8;
@@ -189,3 +203,29 @@ impl Color for HSLColor {
         }
     }
 }
+
+#[cfg(test)]
+mod hue_robustness_tests {
+    use super::*;
+
+    #[test]
+    fn degrees_passed_directly_should_work_for_common_cases() {
+        let red = HSLColor(0.0, 1.0, 0.5).to_backend_color().rgb;
+        assert_eq!(red, (255, 0, 0));
+
+        let green = HSLColor(120.0, 1.0, 0.5).to_backend_color().rgb;
+        assert_eq!(green, (0, 255, 0));
+
+        let blue = HSLColor(240.0, 1.0, 0.5).to_backend_color().rgb;
+        assert_eq!(blue, (0, 0, 255));
+    }
+
+    #[test]
+    fn from_degrees_and_direct_degrees_are_equivalent() {
+        for &deg in &[0.0, 30.0, 60.0, 120.0, 180.0, 240.0, 300.0, 360.0] {
+            let a = HSLColor(deg, 1.0, 0.5).to_backend_color().rgb;
+            let b = HSLColor::from_degrees(deg, 1.0, 0.5).to_backend_color().rgb;
+            assert_eq!(a, b);
+        }
+    }
+}