fix escape for DOT node ids.

The doc says :
 ...
 node_id    :   ID [ port ]
 ...

and later:

An ID is one of the following:
  - Any string of alphabetic characters, underscores or digits, not beginning with a digit;
  - a number [-]?(.[0-9]+ | [0-9]+(.[0-9]*)? );
  - any double-quoted string ("...") possibly containing escaped quotes (\");
  - an HTML string (<...>).

so just quoting the node id seems enough

1 files changed, 2 insertions, 1 deletions

diff --git a/graph.py b/graph.py
index 05f16c1..3ed0902 100644
--- a/graph.py
+++ b/graph.py
@@ -97,7 +97,8 @@ class DotBackend:
         self.emit('%s [%s];' % (normalize_node_id(name), ", ".join(attrs)))
 
 def normalize_node_id(nid):
-    return nid.replace(' ', '_').replace('-', '_')
+    """returns a suitable DOT node id for `nid`"""
+    return '"%s"' % nid
 
 class GraphGenerator:
     def __init__(self, backend):