add test for datetime64[ns]

Author: emilykl

tests/test_optional/test_graph_objs/test_numpy.py

@@ -0,0 +1,16 @@
+from datetime import datetime
+
+import numpy as np
+
+import plotly.graph_objs as go
+
+
+def test_np_ns_datetime():
+    x = [np.datetime64("2025-09-26").astype("datetime64[ns]")]
+    y = [1.23]
+    scatter = go.Scatter(x=x, y=y, mode="markers")
+
+    # x value should be converted to native datetime
+    assert isinstance(scatter.x[0], datetime)
+    # x value should match original numpy value at microsecond precision
+    assert x[0].astype("datetime64[us]").item() == scatter.x[0]