fix: fix rendering documentation

Signed-off-by: Armin Graf <arminhammer@gmail.com>

Author: arminhammer

README.md

@@ -139,7 +139,7 @@ print(yaml_output)
 
 You can see full examples in the [tests/specification](tests/specification) directory
 
-## Generate workflow state machine and graph
+## Generate workflow rendered graph
 
 **Note** Please note that `pip install serverlessworkflow[viz]` needs to be installed in order for this to work. The `viz` feature installs pydot, which supports `graphviz`.
 
@@ -150,13 +150,13 @@ To dot files:
 workflow.render_graph(filename="/tmp/out.dot")
 ```
 
-The following requires `graphviz` to be installed (verify by checking to see if the `dot` binary is on the PATH):
-
-To png files:
+An example rendered png from the dot file:
 ```python
-workflow.render_graph(filename="/tmp/out.png")
+workflow.render_graph(filename="/tmp/out.png", format="png")
 ```
 
+![Workflow Graph](tests/visualization/accumulate-room-readings.png)
+
 # Local development
 
 ## Install dependencies and run tests

tests/visualization/accumulate-room-readings.png

@@ -33,9 +33,6 @@ def test_graphviz_examples(example_file):
     rendered_graph = workflow.render_graph(
         filename=f"tests/visualization/outputs/{example_name}.dot"
     )
-    rendered_graph = workflow.render_graph(
-        filename=f"tests/visualization/outputs/{example_name}.png"
-    )
 
     # Render PNG visualizations for comparison
     fixture_dot = Path(f"tests/visualization/fixtures/{example_name}.dot")
@@ -52,6 +49,19 @@ def test_graphviz_examples(example_file):
         except (subprocess.CalledProcessError, FileNotFoundError) as e:
             print(f"  Warning: Could not render {fixture_png}: {e}")
 
+    output_filename = Path(f"tests/visualization/outputs/{example_name}.dot")
+    if output_filename.exists():
+        output_png = output_filename.with_suffix(".png")
+        try:
+            subprocess.run(
+                ["dot", "-Tpng", str(output_filename), "-o", str(output_png)],
+                check=True,
+                capture_output=True,
+            )
+            print(f"  Generated: {output_png}")
+        except (subprocess.CalledProcessError, FileNotFoundError) as e:
+            print(f"  Warning: Could not render {output_png}: {e}")
+
     with open(f"tests/visualization/fixtures/{example_name}.dot", encoding="utf-8") as f:
         expected_graph = f.read()