Give graph an optional "file" param, like table's.

IkiWiki/Plugin/graphviz.pm

@@ -95,6 +95,14 @@ sub render_graph (\%) {
 sub graph (@) {
 	my %params=@_;
 
+	if (exists $params{file}) {
+		if (! exists $pagesources{$params{file}}) {
+			error gettext("cannot find file");
+		}
+		$params{src} = readfile(srcfile($params{file}));
+		add_depends($params{page}, $params{file});
+	}
+
 	# Support wikilinks in the graph source.
 	my $src=$params{src};
 	$src="" unless defined $src;

doc/ikiwiki/directive/graph.mdwn

@@ -21,6 +21,7 @@ ikiwiki, [[WikiLinks|ikiwiki/wikilink]] can also be used. For example:
 The `graph` directive supports the following parameters:
 
 - `src` - The graphviz source to render.
+* `file` - A file in the wiki containing the graphviz source.
 - `type` - The type of graph to render: `graph` or `digraph`.  Defaults to
   `digraph`.
 - `prog` - The graphviz program to render with: `dot`, `neato`, `fdp`, `twopi`,