MNT: Warn on using pixel marker for scatter()

Improves on matplotlib#11460.

Author: timhoffm

lib/matplotlib/axes/_axes.py

@@ -4933,6 +4933,12 @@ def scatter(self, x, y, s=None, c=None, marker=None, cmap=None, norm=None,
         # load default marker from rcParams
         if marker is None:
             marker = mpl.rcParams['scatter.marker']
+        if marker == ",":
+            _api.warn_external(
+                "The pixel maker ',' is not supported on scatter(). Fallback "
+                "to a finite-sized square, which is necessarily 1 pixel in "
+                "size. Use the square marker 's' to suppress this warning."
+            )
 
         if isinstance(marker, mmarkers.MarkerStyle):
             marker_obj = marker