For non-html output, let sphinx pick the best format.

anntzer · anntzer · commit caedd43f0a02 · 2019-07-03T15:49:10.000+02:00
See https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html?highlight=figure#images We were hardcoding the image format before anyways (for html, things are different because we make the format configurable). This enables one to support third-party builders instead of having to explicitly list them in the template. Admittedly this will now break if someone tries e.g. to do a build without generating figures in *any* of the formats supported by the sphinx builder, whereas previously we would just skip all images; I think that's reasonable though...
diff --git a/lib/matplotlib/sphinxext/plot_directive.py b/lib/matplotlib/sphinxext/plot_directive.py
@@ -348,7 +348,7 @@ def split_code_at_show(text):
 TEMPLATE = """
 {{ source_code }}
 
-{{ only_html }}
+.. only:: html
 
    {% if source_link or (html_show_formats and not multi_image) %}
    (
@@ -384,29 +384,15 @@ def split_code_at_show(text):
       {{ caption }}
    {% endfor %}
 
-{{ only_latex }}
+.. only:: not html
 
    {% for img in images %}
-   {% if 'pdf' in img.formats -%}
-   .. figure:: {{ build_dir }}/{{ img.basename }}.pdf
+   .. figure:: {{ build_dir }}/{{ img.basename }}.*
       {% for option in options -%}
       {{ option }}
       {% endfor %}
 
       {{ caption }}
-   {% endif -%}
-   {% endfor %}
-
-{{ only_texinfo }}
-
-   {% for img in images %}
-   {% if 'png' in img.formats -%}
-   .. image:: {{ build_dir }}/{{ img.basename }}.png
-      {% for option in options -%}
-      {{ option }}
-      {% endfor %}
-
-   {% endif -%}
    {% endfor %}
 
 """
@@ -777,10 +763,6 @@ def run(arguments, content, options, state_machine, state, lineno):
             ':%s: %s' % (key, val) for key, val in options.items()
             if key in ('alt', 'height', 'width', 'scale', 'align', 'class')]
 
-        only_html = ".. only:: html"
-        only_latex = ".. only:: latex"
-        only_texinfo = ".. only:: texinfo"
-
         # Not-None src_link signals the need for a source link in the generated
         # html
         if j == 0 and config.plot_html_show_source_link:
@@ -794,9 +776,6 @@ def run(arguments, content, options, state_machine, state, lineno):
             build_dir=build_dir_link,
             source_link=src_link,
             multi_image=len(images) > 1,
-            only_html=only_html,
-            only_latex=only_latex,
-            only_texinfo=only_texinfo,
             options=opts,
             images=images,
             source_code=source_code,
