diff --git a/brix11/lib/brix11/brix/common/cmds/gendoc/doxygen.rb b/brix11/lib/brix11/brix/common/cmds/gendoc/doxygen.rb
index 31e3837..4cf5cd7 100644
--- a/brix11/lib/brix11/brix/common/cmds/gendoc/doxygen.rb
+++ b/brix11/lib/brix11/brix/common/cmds/gendoc/doxygen.rb
@@ -33,6 +33,10 @@ def has_doxygen?
             !!(Sys.expand('doxygen -v') =~ /\d+\.\d+\.\d+/)
           end
 
+          def has_dot?
+            !!(Sys.expand('dot -V', '', false) =~ /graphviz/)
+          end
+
           public
 
           def generate(config, noredirect)
@@ -42,14 +46,12 @@ def generate(config, noredirect)
             config = config.gsub(/\$\{(\w+)\}/) { |_| resolve_path_var($1) }
             # check for availability of doxygen
             if has_doxygen?
+              opts = { env: { 'BRIX11_HAVE_DOT' => has_dot? ? 'YES' : 'NO'} }
               Sys.in_dir(Exec.get_run_environment('X11_BASE_ROOT')) do
-                args = []
-                args << '-q' unless noredirect
-                args << config
-                Exec.runcmd('doxygen', *args, noredirect ? {} : {silent: true, filter: NullFilter.new})
+                Exec.runcmd('doxygen', config, noredirect ? opts : opts.merge({silent: true, filter: NullFilter.new}))
               end
             else
-              BRIX11.log_warning('No source documentation will be generated as the doxygen tool is not available!')
+              BRIX11.log_error('No source documentation will be generated as the doxygen tool is not available!')
             end
           end
         end
diff --git a/brix11/lib/brix11/brix/common/docs/generate_documentation.rd b/brix11/lib/brix11/brix/common/docs/generate_documentation.rd
index fa4c22f..42ae71b 100644
--- a/brix11/lib/brix11/brix/common/docs/generate_documentation.rd
+++ b/brix11/lib/brix11/brix/common/docs/generate_documentation.rd
@@ -11,6 +11,11 @@ common
 
 === options
 
+  -D, --doxygen                    Generate doxygen source documentation.
+                                   Not generated by default.
+  -N, --no-redirect                Do not redirect output from doxygen process..
+                                   Default: run silent and hide output.
+
   -f, --force                      Force all tasks to run even if their dependencies do not require them to.
                                    Default: off
   -v, --verbose                    Run with increased verbosity level. Repeat to increase more.
@@ -18,9 +23,15 @@ common
 
   -h, --help                       Show this help message.
 
- == Description
+== Description
 
 Generate documentation from Asciidoctor sources into the 'docs' directory. To install
 Asciidoctor with coderay support as gem run the following command
 
 $ gem install asciidoctor:2.0.15 coderay
+
+Installation of the *Doxygen* toolset is required to be able to generate doxygen source documentation.
+When not available an error will be shown and no source documentation is generated.
+
+When the *Graphviz* toolset is installed it will be used by doxygen to generate class inheritance graphs for
+the doxygen source documentation otherwise the (simpler) integrated graph generation will be used.
diff --git a/brix11/lib/brix11/system.rb b/brix11/lib/brix11/system.rb
index aabde84..be4c36a 100644
--- a/brix11/lib/brix11/system.rb
+++ b/brix11/lib/brix11/system.rb
@@ -45,6 +45,8 @@ def self.expand(cmd, default='', redirect_err=true)
       excmd = cmd
       if redirect_err
         excmd << ' 2>' << (mswin? ? 'NUL' : '/dev/null')
+      else
+        excmd << ' 2>&1'
       end
       %x{#{excmd}} rescue default
     end