add line breaks in edge labels if too long

* with many components of the same type, edge description gets much too long in graphs,

* shorten very long items, add line breaks, and even drop items if too many

see issue Fortran-FOSS-Programmers#293
see issue Fortran-FOSS-Programmers#628

ford/graphs.py

@@ -603,6 +603,68 @@ def __init__(self, obj, gd, hist=None):
 def _edge(
     tail: BaseNode, head: BaseNode, style: str, colour: str, label: Optional[str] = None
 ) -> Dict:
+
+    if label != None:
+        len_max = 40
+        nblines_max = 3
+
+        # simple variant with one line abbreviation
+        #if len(label) > len_max:
+        #    label_cut = label[:(len_max-3)] + '...'
+        if len(label) > len_max:
+            label_list = re.split('\s*,\s*', label)
+            nblines = 1
+
+            # avoid special case of first item in loop below
+            l = label_list[0]
+            label_cut = l
+            lencurr = len(l)
+
+            for i in range(1,len(label_list)):
+                l = label_list[i]
+                is_last = (i == len(label_list)-1)
+                if len(l) > (len_max-1):
+                    # first shorten very long names, reserve five characters for either a comma or ', ...'
+                    l = l[:(len_max-3-5)] + '...'
+
+                if (nblines < nblines_max):
+                    # not the last line
+                    # if last item in label_list: use full len_max character length
+                    # if not last item in label_list: reserve 1 character for ',' before linebreak
+                    if ((is_last       and (lencurr + 2 + len(l) > len_max)) or
+                        ((not is_last) and (lencurr + 2 + len(l) > (len_max-1)))):
+                        label_cut = label_cut + ',\n' + l
+                        # new line, reset lencurr to length of current line which contains only l
+                        lencurr = len(l)
+                        nblines = nblines + 1
+                    else:
+                        # item fits onto line
+                        label_cut = label_cut + ', ' + l
+                        # add length of l and ', ' to length of last line
+                        lencurr = lencurr + 2 + len(l)
+
+                else:
+                    # last line, no more linebreaks
+
+                    # if last item in label_list: use full len_max character length
+                    # if not last item in label_list: reserve 5 characters for ', ...'
+                    if ((is_last       and (lencurr + 2 + len(l) > len_max)) or
+                        ((not is_last) and (lencurr + 2 + len(l) > (len_max-5)))):
+                        # abort by finishing the current and last line by ', ...'
+                        label_cut = label_cut + ', ...'
+                        break
+
+                    else:
+                        # item fits onto line
+                        label_cut = label_cut + ', ' + l
+                        # add length of l and ', ' to length of last line
+                        lencurr = lencurr + 2 + len(l)
+
+        else:
+            label_cut = label
+    else:
+        label_cut = None
+
     return {
         "tail_node": tail,
         "head_node": head,
@@ -611,7 +673,7 @@ def _edge(
             "head_name": head.ident,
             "style": style,
             "color": colour,
-            "label": label,
+            "label": label_cut,
         },
     }