diff --git a/default.nix b/default.nix
index f837fc9b..9436fd79 100644
--- a/default.nix
+++ b/default.nix
@@ -83,8 +83,8 @@ let
           xo-reflectutil    = self.callPackage pkgs/xo-reflectutil.nix    {};
           xo-reflect        = self.callPackage pkgs/xo-reflect.nix        {};
           xo-pyreflect      = self.callPackage pkgs/xo-pyreflect.nix      {};
-          xo-ratio          = self.callPackage pkgs/xo-ratio.nix          {};
           xo-unit           = self.callPackage pkgs/xo-unit.nix           {};
+          xo-ratio          = self.callPackage pkgs/xo-ratio.nix          { buildDocs = true; };
           xo-pyunit         = self.callPackage pkgs/xo-pyunit.nix         {};
 #
           xo-callback       = self.callPackage pkgs/xo-callback.nix       {};
diff --git a/pkgs/xo-ratio.nix b/pkgs/xo-ratio.nix
index 8c55ed78..35acd4fb 100644
--- a/pkgs/xo-ratio.nix
+++ b/pkgs/xo-ratio.nix
@@ -1,13 +1,14 @@
 {
   # dependencies
-  stdenv, cmake, #catch2,
-  doxygen,
+  lib, stdenv, cmake, #catch2,
 
   python3Packages,
 
-  sphinx,
+  doxygen, sphinx, graphviz,
 
   xo-cmake, xo-flatstring, xo-reflectutil,
+
+  buildDocs ? false,
 } :
 
 stdenv.mkDerivation (finalattrs: