fix: DH-18685: Remove top margin from chart (#1126)

Plotly Express adds a top margin that I have now removed. Fixes
[DH-18685](https://deephaven.atlassian.net/issues/DH-18685?jql=ORDER%20BY%20created%20DESC)

```
from deephaven import empty_table, ui
from deephaven.plot import express as dx

t = empty_table(10).update(["x=i", "y=i"])
p = dx.line(t, x="x", y="y", title="Title")
p2 = dx.line(t, x="x", y="y")
```

[DH-18685]:
https://deephaven.atlassian.net/browse/DH-18685?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: jnumainville

plugins/plotly-express/src/deephaven/plot/express/deephaven_figure/generate.py

@@ -1067,6 +1067,14 @@ def generate_figure(
         extra_generators=[hover_text],
     )
 
+    # px adds a margin of 60 if a title is not specified
+    # since most charts still use px at their core and
+    # this isn't user controlled in any way, remove it after
+    # the figure is already created
+    px_fig.update_layout(
+        margin_t=None,
+    )
+
     dh_fig = DeephavenFigure(
         px_fig,
         call_args=call_args,

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

@@ -69,7 +69,6 @@ def test_basic_area(self):
                 "side": "left",
             },
             "legend": {"tracegroupgap": 0},
-            "margin": {"t": 60},
         }
 
         self.assertEqual(plotly["layout"], expected_layout)
@@ -134,7 +133,6 @@ def test_area_step(self):
                 "side": "left",
             },
             "legend": {"tracegroupgap": 0},
-            "margin": {"t": 60},
         }
 
         self.assertEqual(plotly["layout"], expected_layout)
@@ -191,7 +189,6 @@ def test_area_pandas(self):
                 "side": "left",
             },
             "legend": {"tracegroupgap": 0},
-            "margin": {"t": 60},
         }
 
         expected_mappings = [

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

@@ -55,7 +55,6 @@ def test_basic_bar_x(self):
         expected_layout = {
             "barmode": "relative",
             "legend": {"tracegroupgap": 0},
-            "margin": {"t": 60},
             "xaxis": {
                 "anchor": "y",
                 "domain": [0.0, 1.0],
@@ -121,7 +120,6 @@ def test_basic_bar_y(self):
         expected_layout = {
             "barmode": "relative",
             "legend": {"tracegroupgap": 0},
-            "margin": {"t": 60},
             "xaxis": {
                 "anchor": "y",
                 "domain": [0.0, 1.0],
@@ -187,7 +185,6 @@ def test_basic_bar_x_y(self):
         expected_layout = {
             "barmode": "relative",
             "legend": {"tracegroupgap": 0},
-            "margin": {"t": 60},
             "xaxis": {
                 "anchor": "y",
                 "domain": [0.0, 1.0],