diff --git a/sql/Makefile b/sql/Makefile
index 4081f85e..eef10a16 100644
--- a/sql/Makefile
+++ b/sql/Makefile
@@ -1,52 +1,53 @@
 # Depends: postgresql-client, postgresql-autodoc
 
 DBNAME = softwareheritage-dev
 DOCDIR = autodoc
 
 SQL_INIT   = swh-init.sql
 SQL_SCHEMA = swh-schema.sql
 SQL_FUNC   = swh-func.sql
 SQL_DATA   = swh-data.sql
 SQLS = $(SQL_INIT) $(SQL_SCHEMA) $(SQL_FUNC) $(SQL_DATA)
 
 PSQL_BIN = psql
 PSQL_FLAGS = --single-transaction --echo-all
 PSQL = $(PSQL_BIN) $(PSQL_FLAGS)
 
 
 all:
 
 createdb: createdb-stamp
 createdb-stamp: $(SQL_INIT)
 	createdb $(DBNAME)
 	touch $@
 
 filldb: filldb-stamp
 filldb-stamp: createdb-stamp
 	cat $(SQLS) | $(PSQL) $(DBNAME)
 	touch $@
 
 dropdb:
 	-dropdb $(DBNAME)
 
 dumpdb: swh.dump
 swh.dump: filldb-stamp
 	pg_dump -Fc $(DBNAME) > $@
 
 doc: autodoc-stamp $(DOCDIR)/swh.pdf
 autodoc-stamp: filldb-stamp
 	test -d $(DOCDIR)/ || mkdir $(DOCDIR)
 	postgresql_autodoc -d $(DBNAME) -f $(DOCDIR)/swh
+	cp -a $(DOCDIR)/swh.dot $(DOCDIR)/swh.dot.orig
 	touch $@
 
 $(DOCDIR)/swh.pdf: clusters.dot autodoc-stamp
-	bin/dot_add_content $(DOCDIR)/swh.dot clusters.dot
+	bin/dot_add_content $(DOCDIR)/swh.dot.orig clusters.dot > $(DOCDIR)/swh.dot
 	dot -T pdf $(DOCDIR)/swh.dot > $(DOCDIR)/swh.pdf
 
 clean:
 	rm -rf *-stamp $(DOCDIR)/
 
 distclean: clean dropdb
 	rm -f swh.dump
 
 .PHONY: all initdb createdb dropdb doc clean
diff --git a/sql/bin/dot_add_content b/sql/bin/dot_add_content
index 679fdf31..e0f507b3 100755
--- a/sql/bin/dot_add_content
+++ b/sql/bin/dot_add_content
@@ -1,20 +1,12 @@
 #!/bin/bash
 
-# Depends: moreutils
-
 DOT_FILE="$1"
 DOT_EXTRA="$2"
 if [ -z "$DOT_FILE" -o -z "$DOT_EXTRA" ] ; then
     echo "Usage: $0 DOT_FILE DOT_EXTRA"
     exit 1
 fi
 
-if ! [ -x $(which sponge) ] ; then
-    echo "Can't find sponge. Please: apt-get install moreutils"
-    exit 1
-fi
-
-(head -n -1 "$DOT_FILE" ;
- cat "$DOT_EXTRA" ;
- echo "}") \
-    | sponge "$DOT_FILE"
+head -n -1 "$DOT_FILE"  # all of $DOT_FILE but last line
+cat "$DOT_EXTRA"
+echo "}"
diff --git a/sql/clusters.dot b/sql/clusters.dot
index ac34985d..42d1e160 100644
--- a/sql/clusters.dot
+++ b/sql/clusters.dot
@@ -1,64 +1,72 @@
-subgraph cluster_meta {
-	label = "Meta";
-	dbversion;
-}
 
-subgraph cluster_content {
-	label = "Content";
-	content;
-}
+subgraph "logical_grouping" {
+	style = rounded;
+	bgcolor = gray95;
+	color = gray;
+	
+	subgraph cluster_meta {
+		label = <<b>meta</b>>;
+		dbversion;
+	}
 
-subgraph cluster_directory {
-	label = "Directories";
-	directory;
-	directory_entry_dir;
-	directory_entry_file;
-	directory_entry_rev;
-	directory_list_dir;
-	directory_list_file;
-	directory_list_rev;
-}
+	subgraph cluster_content {
+		label = <<b>content</b>>;
+		content;
+	}
 
-subgraph cluster_revision {
-	label = "Revisions";
-	revision;
-	revision_history;
-	person;
-}
+	subgraph cluster_directory {
+		label = <<b>directories</b>>;
+		directory;
+		directory_entry_dir;
+		directory_entry_file;
+		directory_entry_rev;
+		directory_list_dir;
+		directory_list_file;
+		directory_list_rev;
+	}
 
-subgraph cluster_release {
-	label = "Releases";
-	release;
-}
+	subgraph cluster_revision {
+		label = <<b>revisions</b>>;
+		revision;
+		revision_history;
+		person;
+	}
 
-subgraph cluster_occurrences {
-	label = "Occurrences";
-	occurrence;
-	occurrence_history;
-}
+	subgraph cluster_release {
+		label = <<b>releases</b>>;
+		release;
+	}
 
-subgraph cluster_origins {
-	label = "Origins";
-	origin;
-	fetch_history;
-}
+	subgraph cluster_occurrences {
+		label = <<b>occurrences</b>>;
+		occurrence;
+		occurrence_history;
+	}
 
-subgraph cluster_projects {
-	label = "Projects";
-	project;
-	project_history;
-}
+	subgraph cluster_origins {
+		label = <<b>origins</b>>;
+		origin;
+		fetch_history;
+	}
 
-subgraph cluster_organization {
-	label = "Organizations";
-	organization;
-	list_history;
-}
+	subgraph cluster_projects {
+		label = <<b>projects</b>>;
+		project;
+		project_history;
+	}
+
+	subgraph cluster_organization {
+		label = <<b>organizations</b>>;
+		organization;
+		list_history;
+	}
+
+	{
+		edge [style = dashed];
 
-{
-	edge [style = dashed];
+		directory_entry_file -> content;
+		directory_entry_rev -> revision;
+		revision -> directory;
+	}
 
-	directory_entry_file -> content;
-	directory_entry_rev -> revision;
-	revision -> directory;
 }