In imsave, let pnginfo have precedence over metadata.

anntzer · anntzer · commit e9891e2c02c3 · 2019-10-17T15:14:37.000+02:00
This is consistent with the documented behavior ("If the 'pnginfo' key
is present, it completely overrides *metadata*, including the default
'Software' key.") and also with the behavior of
FigureCanvasAgg.print_png.
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -1539,7 +1539,8 @@ def imsave(fname, arr, vmin=None, vmax=None, cmap=None, format=None,
             pil_shape = (rgba.shape[1], rgba.shape[0])
             image = Image.frombuffer(
                 "RGBA", pil_shape, rgba, "raw", "RGBA", 0, 1)
-            if format == "png" and metadata is not None:
+            if (format == "png"
+                    and metadata is not None and "pnginfo" not in pil_kwargs):
                 # cf. backend_agg's print_png.
                 pnginfo = PngInfo()
                 for k, v in metadata.items():
