[svg] Add rcParam["svg.id"] to add a top-level id attribute to <svg> (matplotlib#28536)

* (backend.svg) add `svg.id` rcParam

If not None, the `svg.id` rcParam value will be used for setting
the `id` attribute of the top `<svg>` tag.

* Update doc/users/next_whats_new/svg_id_rc.rst

* Update doc/users/next_whats_new/svg_id_rc.rst

---------

Co-authored-by: Kherim Willems <[email protected]>
Co-authored-by: Tim Hoffmann <[email protected]>

Author: 3 people

doc/users/next_whats_new/svg_id_rc.rst

@@ -0,0 +1,32 @@
+``svg.id`` rcParam
+~~~~~~~~~~~~~~~~~~
+:rc:`svg.id` lets you insert an ``id`` attribute into the top-level ``<svg>`` tag.
+
+e.g. ``rcParams["svg.id"] = "svg1"`` results in
+default), no ``id`` tag is included
+
+.. code-block:: XML
+
+    <svg
+        xmlns:xlink="http://www.w3.org/1999/xlink"
+        width="50pt" height="50pt"
+        viewBox="0 0 50 50"
+        xmlns="http://www.w3.org/2000/svg"
+        version="1.1"
+        id="svg1"
+    ></svg>
+
+This is useful if you would like to link the entire matplotlib SVG file within
+another SVG file with the ``<use>`` tag.
+
+.. code-block:: XML
+
+    <svg>
+    <use
+        width="50" height="50"
+        xlink:href="mpl.svg#svg1" id="use1"
+        x="0" y="0"
+    /></svg>
+
+Where the ``#svg1`` indicator will now refer to the top level ``<svg>`` tag, and
+will hence result in the inclusion of the entire file.

lib/matplotlib/backends/backend_svg.py

@@ -322,6 +322,7 @@ def __init__(self, width, height, svgwriter, basename=None, image_dpi=72,
             viewBox=f'0 0 {str_width} {str_height}',
             xmlns="http://www.w3.org/2000/svg",
             version="1.1",
+            id=mpl.rcParams['svg.id'],
             attrib={'xmlns:xlink': "http://www.w3.org/1999/xlink"})
         self._write_metadata(metadata)
         self._write_default_style()

lib/matplotlib/mpl-data/matplotlibrc

@@ -735,6 +735,8 @@
                          #     None: Assume fonts are installed on the
                          #           machine where the SVG will be viewed.
 #svg.hashsalt: None      # If not None, use this string as hash salt instead of uuid4
+#svg.id: None            # If not None, use this string as the value for the `id`
+                         # attribute in the top <svg> tag
 
 ### pgf parameter
 ## See https://matplotlib.org/stable/tutorials/text/pgf.html for more information.

lib/matplotlib/rcsetup.py

@@ -1311,6 +1311,7 @@ def _convert_validator_spec(key, conv):
     "svg.image_inline": validate_bool,
     "svg.fonttype": ["none", "path"],  # save text as text ("none") or "paths"
     "svg.hashsalt": validate_string_or_None,
+    "svg.id": validate_string_or_None,
 
     # set this when you want to generate hardcopy docstring
     "docstring.hardcopy": validate_bool,

lib/matplotlib/tests/test_backend_svg.py

@@ -669,3 +669,34 @@ def test_annotationbbox_gid():
 
     expected = '<g id="a test for issue 20044">'
     assert expected in buf
+
+
+def test_svgid():
+    """Test that `svg.id` rcparam appears in output svg if not None."""
+
+    fig, ax = plt.subplots()
+    ax.plot([1, 2, 3], [3, 2, 1])
+    fig.canvas.draw()
+
+    # Default: svg.id = None
+    with BytesIO() as fd:
+        fig.savefig(fd, format='svg')
+        buf = fd.getvalue().decode()
+
+    tree = xml.etree.ElementTree.fromstring(buf)
+
+    assert plt.rcParams['svg.id'] is None
+    assert not tree.findall('.[@id]')
+
+    # String: svg.id = str
+    svg_id = 'a test for issue 28535'
+    plt.rc('svg', id=svg_id)
+
+    with BytesIO() as fd:
+        fig.savefig(fd, format='svg')
+        buf = fd.getvalue().decode()
+
+    tree = xml.etree.ElementTree.fromstring(buf)
+
+    assert plt.rcParams['svg.id'] == svg_id
+    assert tree.findall(f'.[@id="{svg_id}"]')