fix: Ensure title is added to default figure

plugins/plotly-express/src/deephaven/plot/express/plots/PartitionManager.py

@@ -603,7 +603,9 @@ def default_figure(self) -> DeephavenFigure:
         # this is very hacky but it's needed to prevent errors when
         # there are no partitions until a better solution can be done
         # also need the px template to be set
-        default_fig = px.scatter(x=[0], y=[0])
+        # the title can also be set here as it will never change
+        title = self.args.get("title")
+        default_fig = px.scatter(x=[0], y=[0], title=title)
         default_fig.update_traces(x=[], y=[])
         return DeephavenFigure(default_fig)
 

plugins/plotly-express/test/deephaven/plot/express/plots/test_layout.py

@@ -0,0 +1,26 @@
+import unittest
+
+from ..BaseTest import BaseTestCase
+
+
+class ScatterTestCase(BaseTestCase):
+    def setUp(self) -> None:
+        from deephaven import empty_table
+
+        self.source = empty_table(0).update(["x=i", "y=i*Math.random()*i", "z=i%5"])
+
+    def test_empty_title(self):
+        import src.deephaven.plot.express as dx
+
+        chart = dx.line(
+            self.source, x="x", y="y", by=["z"], title="Hello World0"
+        ).to_dict(self.exporter)
+        plotly, deephaven = chart["plotly"], chart["deephaven"]
+
+        expected_title = {"text": "Hello World0"}
+
+        self.assertEqual(plotly["layout"]["title"], expected_title)
+
+
+if __name__ == "__main__":
+    unittest.main()