diff --git a/Graphics/Diagram/Plotter.hs b/Graphics/Diagram/Plotter.hs
index b7d4ed0..2ffc1e7 100644
--- a/Graphics/Diagram/Plotter.hs
+++ b/Graphics/Diagram/Plotter.hs
@@ -189,6 +189,7 @@ squares = Diag f
 
 
 
+-- |Get the quad tree corresponding to the given points and diagram properties.
 qt :: [PT] -> DiagProp -> QuadTree PT
 qt vt p = quadTree (filterValidPT p vt) (dX p, dY p)