graph: Add '%referrer-node-type'.

* guix/scripts/graph.scm (ensure-store-items): New procedure.
(%reference-node-type)[convert]: Use it.
(non-derivation-referrers): New procedure.
(%referrer-node-type): New variable.
(%node-types): Add it.
* tests/graph.scm ("referrer DAG"): New test.
* doc/guix.texi (Invoking guix graph): Document it.

1 files changed, 14 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 74733f4fd1..47fc199c6c 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5546,6 +5546,20 @@ example, the command below produces the reference graph of your profile
 @example
 guix graph -t references `readlink -f ~/.guix-profile`
 @end example
+
+@item referrers
+This is the graph of the @dfn{referrers} of a store item, as returned by
+@command{guix gc --referrers} (@pxref{Invoking guix gc}).
+
+This relies exclusively on local information from your store.  For
+instance, let us suppose that the current Inkscape is available in 10
+profiles on your machine; @command{guix graph -t referrers inkscape}
+will show a graph rooted at Inkscape and with those 10 profiles linked
+to it.
+
+It can help determine what is preventing a store item from being garbage
+collected.
+
 @end table
 
 The available options are the following: