Fix missing labels (#18356)

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- [ ] **PR title**: "package: description"
- Where "package" is whichever of langchain, community, core,
experimental, etc. is being modified. Use "docs: ..." for purely docs
changes, "templates: ..." for template changes, "infra: ..." for CI
changes.
  - Example: "community: add foobar LLM"


- [ ] **PR message**: ***Delete this entire checklist*** and replace
with
    - **Description:** a description of the change
    - **Issue:** the issue # it fixes, if applicable
    - **Dependencies:** any dependencies required for this change
- **Twitter handle:** if your PR gets announced, and you'd like a
mention, we'll gladly shout you out!


- [ ] **Add tests and docs**: If you're adding a new integration, please
include
1. a test for the integration, preferably unit tests that do not rely on
network access,
2. an example notebook showing its use. It lives in
`docs/docs/integrations` directory.


- [ ] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, hwchase17.

libs/core/langchain_core/runnables/graph.py

@@ -263,4 +263,17 @@ class Graph:
     ) -> Union[bytes, None]:
         from langchain_core.runnables.graph_png import PngDrawer
 
-        return PngDrawer(fontname, labels).draw(self, output_file_path)
+        default_node_labels = {
+            node.id: node_data_str(node) for node in self.nodes.values()
+        }
+
+        return PngDrawer(
+            fontname,
+            LabelsDict(
+                nodes={
+                    **default_node_labels,
+                    **(labels["nodes"] if labels is not None else {}),
+                },
+                edges=labels["edges"] if labels is not None else {},
+            ),
+        ).draw(self, output_file_path)