feat: update SVG handling in AltairFormatter to use io_to_data_url for data URL encoding

Author: daizutabi

marimo/_output/formatters/altair_formatters.py

@@ -1,7 +1,6 @@
 # Copyright 2026 Marimo. All rights reserved.
 from __future__ import annotations
 
-import base64
 import json
 from typing import Any
 from urllib.request import urlopen
@@ -18,7 +17,6 @@
     get_chart_mimetype,
     maybe_fix_vegafusion_background,
 )
-from marimo._utils.data_uri import build_data_url
 
 LOGGER = marimo_logger()
 
@@ -83,8 +81,9 @@ def _show_chart(chart: AltairChartType) -> tuple[KnownMimeType, str]:
                         return (mime_type, data_url or "")
                     if isinstance(mime_response, str):
                         if mime_type == "image/svg+xml":
-                            data = base64.b64encode(mime_response.encode())
-                            return mime_type, build_data_url(mime_type, data)
+                            svg_bytes = mime_response.encode()
+                            data_url = io_to_data_url(svg_bytes, mime_type)
+                            return (mime_type, data_url or "")
                         return mime_type, mime_response
                     return mime_type, json.dumps(mime_response)