diff --git a/tools/transform.rs b/tools/transform.rs
index 8f9b59c..4f6f5e8 100644
--- a/tools/transform.rs
+++ b/tools/transform.rs
@@ -242,7 +242,10 @@ fn from_polar(coord: Coord) -> Coord {
 
 fn to_polar(coord: Coord) -> Coord {
     let r = f64::sqrt(coord.x.powi(2) + coord.y.powi(2));
-    let theta = f64::atan2(coord.y, coord.x);
+    let mut theta = f64::atan2(coord.y, coord.x);
+    if theta < 0.0 {
+        theta += 2.0 * std::f64::consts::PI;
+    }
     coord! { x: r, y: theta}
 }