Mention minimum Matplotlib versions for different formats and fix test suite

Author: astrofrog

README.rst

@@ -286,9 +286,10 @@ in ``mpl_image_compare``. Supported formats are ``'eps'``, ``'pdf'``, ``'png'``,
 Note that Ghostscript is required to be installed for comparing PDF and EPS figures, while
 Inkscape is required for SVG comparison.
 
-By default, Matplotlib does not produce deterministic output that will
-have a consistent hash every time it is run, or over different Matplotlib versions.
-In order to enforce that the output is deterministic, you will need to do the following:
+By default, Matplotlib does not produce deterministic output that will have a
+consistent hash every time it is run, or over different Matplotlib versions. In
+order to enforce that the output is deterministic, you will need to set metadata
+as described in the following subsections.
 
 PNG
 ^^^
@@ -311,6 +312,8 @@ For PDF files, the output can be made deterministic by setting:
                                                                 "Producer": None,
                                                                 "CreationDate": None}})
 
+Note that deterministic PDF output can only be achieved with Matplotlib 2.1 and above
+
 EPS
 ^^^
 
@@ -330,6 +333,8 @@ a constant value (this is a unit timestamp):
 
 You could do this inside the test.
 
+Note that deterministic PDF output can only be achieved with Matplotlib 2.1 and above
+
 SVG
 ^^^
 
@@ -345,6 +350,8 @@ and in addition, you should make sure the following rcParam is set to a constant
 
     plt.rcParams['svg.hashsalt'] = 'test'
 
+Note that SVG files can only be used in pytest-mpl with Matplotlib 3.3 and above.
+
 Test failure example
 --------------------
 

tests/baseline/hashes/mpl20_ft261.json

@@ -13,7 +13,5 @@
   "test.test_modified": "54f6cf83d5b06fa2ecb7fa23d6e87898679178ef5d0dfdd2551a139f1932127b",
   "test.test_new": "54f6cf83d5b06fa2ecb7fa23d6e87898679178ef5d0dfdd2551a139f1932127b",
   "test.test_unmodified": "54f6cf83d5b06fa2ecb7fa23d6e87898679178ef5d0dfdd2551a139f1932127b",
-  "test_formats.test_format_eps": "d43d276873ee3a46e4dfc86feba5b814a76fa72163dfc6e9624a2750ff625f2f",
-  "test_formats.test_format_pdf": "917a633ed43b288ecd4ccad64c2f61eee5f2a1b6383a808572dd5a63671298f6",
   "test_formats.test_format_png": "480062c2239ed9d70e361d1a5b578dc2aa756971161ac6e7287b492ae6118c59"
 }

tests/test_pytest_mpl.py

@@ -685,8 +685,15 @@ def test_formats(pytester, use_hash_library, passes, file_format):
     if file_format == 'svg' and MPL_VERSION < Version('3.3'):
         pytest.skip('SVG comparison is only supported in Matplotlib 3.3 and above')
 
-    if use_hash_library and MPL_VERSION >= Version('3.4'):
-        pytest.skip('No hash library for Matplotlib >= 3.4')
+    if use_hash_library:
+
+        if file_format == 'pdf' and MPL_VERSION < Version('2.1'):
+            pytest.skip('PDF hashes are only deterministic in Matplotlib 2.1 and above')
+        elif file_format == 'eps' and MPL_VERSION < Version('2.1'):
+            pytest.skip('EPS hashes are only deterministic in Matplotlib 2.1 and above')
+
+        if MPL_VERSION >= Version('3.4'):
+            pytest.skip('No hash library in test suite for Matplotlib >= 3.4')
 
     if use_hash_library and not sys.platform.startswith('linux'):
         pytest.skip('Hashes for vector graphics are only provided in the hash library for Linux')