feat(charts): support palette indices for AreaFill colors

Update AreaFill model to accept both hex strings and palette indices for
color fields. Changes include:

- Allow `color` field to accept int (palette index) or str (hex color)
- Change default `color` from "#15607a" to 0 (first palette color)
- Allow `color_negative` to accept int, str, or None
- Change default `color_negative` from "#cc0000" to None (disabled)
- Update auto-enable logic to check for None instead of default hex value
- Only serialize `colorNegative` when explicitly set (not None)
- Update field descriptions to reflect new behavior

This provides more flexibility for users to reference theme palette colors
by index while maintaining backward compatibility with hex color strings.

Author: palewire

datawrapper/charts/annos.py

@@ -321,8 +321,10 @@ class AreaFill(BaseModel):
     #: The line to fill upwards to
     to_column: str = Field(alias="to", description="The line to fill upwards to")
 
-    #: The color of the fill
-    color: str = Field(default="#15607a", description="The color of the fill")
+    #: The color of the fill (hex string or palette index)
+    color: str | int = Field(
+        default=0, description="The color of the fill (hex string or palette index)"
+    )
 
     #: The opacity of the fill
     opacity: float = Field(default=0.3, description="The opacity of the fill")
@@ -334,11 +336,11 @@ class AreaFill(BaseModel):
         description="Whether to use different colors when there are negative values",
     )
 
-    #: The color of the fill when it is negative
-    color_negative: str = Field(
-        default="#cc0000",
+    #: The color of the fill when it is negative (hex string or palette index, None = disabled)
+    color_negative: str | int | None = Field(
+        default=None,
         alias="colorNegative",
-        description="The color of the fill when it is negative",
+        description="The color of the fill when it is negative (hex string or palette index, None = disabled)",
     )
 
     #: The interpolation method to use when drawing lines
@@ -364,29 +366,35 @@ def validate_interpolation(
     def auto_enable_mixed_colors(self) -> "AreaFill":
         """Auto-enable use_mixed_colors when color_negative is provided.
 
-        If a user provides a color_negative value (different from the default),
+        If a user provides a color_negative value (not None),
         automatically enable use_mixed_colors to make the feature work as expected.
         """
-        # Only auto-enable if color_negative differs from default and use_mixed_colors is False
-        if self.color_negative != "#cc0000" and not self.use_mixed_colors:
+        # Only auto-enable if color_negative is provided and use_mixed_colors is False
+        if self.color_negative is not None and not self.use_mixed_colors:
             self.use_mixed_colors = True
         return self
 
     def serialize_model(self) -> dict:
         """Serialize the model to a dictionary for the Datawrapper API.
 
         Note: The 'id' field is not included in the output as it's used as the dict key.
+        Only includes colorNegative if it's not None.
         """
-        return {
+        result = {
             "from": self.from_column,
             "to": self.to_column,
             "color": self.color,
             "opacity": self.opacity,
             "useMixedColors": self.use_mixed_colors,
-            "colorNegative": self.color_negative,
             "interpolation": self.interpolation,
         }
 
+        # Only include colorNegative if it's provided (not None)
+        if self.color_negative is not None:
+            result["colorNegative"] = self.color_negative
+
+        return result
+
     @classmethod
     def deserialize_model(cls, api_data: dict[str, dict] | list | None) -> list[dict]:
         """Deserialize area fills from API response format.