create cache directory

This is an use case I haven't thought about when closing #2157.

Do you think it's worth to add an option for that, like suggested in the above ticket, or should we just go ahead and create the folder if it's missing?

i'd consider an option for this overkill.

I vote for just creating the directory

I suggest something like the following which is working for us:

Index: graphviz/graphviz.py
===================================================================
--- graphviz/graphviz.py        (revision 6365)
+++ graphviz/graphviz.py        (working copy)
@@ -456,11 +456,11 @@
             self.cache_dir = os.path.join(self.env.path, self.cache_dir)

         if not os.path.exists(self.cache_dir):
-            if self.cache_dir_option == self.DEFAULT_CACHE_DIR:
-                os.mkdir(self.cache_dir)
-            else:
-                return _("The cache_dir '%(path)s' doesn't exist, "
-                         "please create it.", path=self.cache_dir)
+            #if self.cache_dir_option == self.DEFAULT_CACHE_DIR:
+            try:
+                os.makedirs(self.cache_dir)
+            except Exception, e:
+                return _("The cache_dir '%(path)s' doesn't exist, and we failed to create it please create it. :%(e)s", path=self.cache_dir, e=e)

         # Get optional configuration parameters from trac.ini.

#5409 closed as a duplicate.

I've just implemented this same feature for the TracPlantUmlPlugin ([11255]), in the way described by bobbysmith007 in comment:4. I'd like to go ahead and implement the same for this plugin. If no objection from the author / maintainer, or no response for two weeks in accordance with AdoptingHacks, I will proceed.

I added an option to specify the cache directory in [11255], but after reading the comments in this ticket, I agree that an option is overkill. I'd propose just creating the directory at $TRACENV/cache/graphviz.