Delete tweak.override attributes with null value

docs/syntax.md

@@ -345,10 +345,11 @@ Alternatively items can be added to just the BOM by putting them in the section
     # Entries with an attribute containing HTML are
     # not supported.
     <str>:  # leading string of .gv entry
-      <str> : <str>  # attribute and its new value
+      <str> : <str/null>  # attribute and its new value
       # Any number of attributes can be overridden
       # for each entry. Attributes not already existing
       # in the entry will be appended to the entry.
+      # Use null as new value to delete an attribute.
 
   append: <str/list> # string or list of strings to append to the .gv output
 ```

src/wireviz/Harness.py

@@ -357,7 +357,7 @@ def typecheck(name: str, value: Any, expect: type) -> None:
                 typecheck(f'tweak.override.{k} value', d, dict)
                 for a, v in d.items():
                     typecheck(f'tweak.override.{k}.{a} key', a, str)
-                    typecheck(f'tweak.override.{k}.{a} value', v, str)
+                    typecheck(f'tweak.override.{k}.{a} value', v, (str, type(None)))
 
             # Override generated attributes of selected entries matching tweak.override.
             for i, entry in enumerate(dot.body):
@@ -367,7 +367,14 @@ def typecheck(name: str, value: Any, expect: type) -> None:
                     keyword = match and match[2]
                     if keyword in self.tweak.override.keys():
                         for attr, value in self.tweak.override[keyword].items():
-                            # TODO?: If value is None: delete attr?
+                            if value is None:
+                                entry, n_subs = re.subn(f'( +)?{attr}=("[^"]*"|[^] ]*)(?(1)| *)', '', entry)
+                                if n_subs < 1:
+                                    print(f'Harness.create_graph() warning: {attr} not found in {keyword}!')
+                                elif n_subs > 1:
+                                    print(f'Harness.create_graph() warning: {attr} removed {n_subs} times in {keyword}!')
+                                continue
+
                             if len(value) == 0 or ' ' in value:
                                 value = value.replace('"', r'\"')
                                 value = f'"{value}"'
@@ -377,6 +384,7 @@ def typecheck(name: str, value: Any, expect: type) -> None:
                                 entry = re.sub(r'\]$', f' {attr}={value}]', entry)
                             elif n_subs > 1:
                                 print(f'Harness.create_graph() warning: {attr} overridden {n_subs} times in {keyword}!')
+
                         dot.body[i] = entry
 
         if self.tweak.append is not None: