Fix/issue 54204

doc/source/getting_started/intro_tutorials/04_plotting.rst

@@ -108,7 +108,7 @@ I want to visually compare the :math:`NO_2` values measured in London versus Par
    :okwarning:
 
     @savefig 04_airqual_scatter.png
-    air_quality.plot.scatter(x="station_london", y="station_paris", alpha=0.5)
+    air_quality.plot.scatter(x="station_london", y="station_paris", s = 20, alpha=0.5)
     plt.show()
 
 .. raw:: html
@@ -121,6 +121,7 @@ number of alternatives are available to plot data. Let’s use some
 standard Python to get an overview of the available plot methods:
 
 .. ipython:: python
+    :okwarning:
 
     [
         method_name

doc/source/user_guide/dsintro.rst

@@ -577,7 +577,7 @@ greater than 5, calculate the ratio, and plot:
            SepalRatio=lambda x: x.SepalWidth / x.SepalLength,
            PetalRatio=lambda x: x.PetalWidth / x.PetalLength,
        )
-       .plot(kind="scatter", x="SepalRatio", y="PetalRatio")
+       .plot(kind="scatter", x="SepalRatio", y="PetalRatio", s = 20)
    )
 
 Since a function is passed in, the function is computed on the DataFrame

doc/source/user_guide/visualization.rst

@@ -626,7 +626,7 @@ It is recommended to specify ``color`` and ``label`` keywords to distinguish eac
 
    ax = df.plot.scatter(x="a", y="b", color="DarkBlue", label="Group 1")
    @savefig scatter_plot_repeated.png
-   df.plot.scatter(x="c", y="d", color="DarkGreen", label="Group 2", ax=ax);
+   df.plot.scatter(x="c", y="d", color="DarkGreen", label="Group 2", ax=ax, s = 20);
 
 .. ipython:: python
    :suppress:

doc/source/whatsnew/v2.1.0.rst

@@ -785,6 +785,7 @@ Plotting
 ^^^^^^^^
 - Bug in :meth:`Series.plot` when invoked with ``color=None`` (:issue:`51953`)
 - Fixed UserWarning in :meth:`DataFrame.plot.scatter` when invoked with ``c="b"`` (:issue:`53908`)
+- Fixed bug in :meth:`DataFrame.plot.scatter` wherein marker size was previously hardcoded to a default value (:issue:`54204`)
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/plotting/_matplotlib/core.py

@@ -23,7 +23,10 @@
 import numpy as np
 
 from pandas._libs import lib
-from pandas.errors import AbstractMethodError
+from pandas.errors import (
+    AbstractMethodError,
+    PandasFutureWarning,
+)
 from pandas.util._decorators import cache_readonly
 from pandas.util._exceptions import find_stack_level
 
@@ -1325,8 +1328,17 @@ def __init__(
         **kwargs,
     ) -> None:
         if s is None:
-            # hide the matplotlib default for size, in case we want to change
-            # the handling of this argument later
+            # The default size of the elements in a scatter plot
+            # is 20, but this will change in a future version.
+            # In the future the value will be derived from
+            # mpl.rcParams["lines.markersize"] if not provided
+            warnings.warn(
+                "The default of s=20 will be changed to use "
+                "mpl.rcParams['lines.markersize'] in the future. "
+                "Specify `s` to suppress this warning",
+                PandasFutureWarning,
+                stacklevel=find_stack_level(),
+            )
             s = 20
         elif is_hashable(s) and s in data.columns:
             s = data[s]

pandas/tests/plotting/test_misc.py

@@ -5,6 +5,7 @@
 import numpy as np
 import pytest
 
+from pandas.errors import PandasFutureWarning
 import pandas.util._test_decorators as td
 
 from pandas import (
@@ -864,3 +865,45 @@ def test_plot_bar_label_count_expected_success():
         [(30, 10, 10, 10), (20, 20, 20, 20), (10, 30, 30, 10)], columns=list("ABCD")
     )
     df.plot(subplots=[("A", "B", "D")], kind="bar", title=["A&B&D", "C"])
+
+
+def test_change_scatter_markersize_future_warning():
+    # GH 54204
+    # Will raise FutureWarning if s not provided to df.plot.scatter
+    df = DataFrame(data={"x": [1, 2, 3], "y": [1, 2, 3]})
+    with tm.assert_produces_warning(PandasFutureWarning):
+        pandas_default_without_rcparams = df.plot.scatter(
+            x="x", y="y", title="pandas scatter, default rc marker size"
+        )
+    # Verify that pandas still defaults to 20
+    assert pandas_default_without_rcparams.collections[0].get_sizes()[0] == 20
+
+
+@pytest.mark.filterwarnings(
+    "ignore:The default of s=20 will be changed:pandas.errors.PandasFutureWarning"
+)
+def test_scatter_markersize_same_default_with_rcparams():
+    # GH 54204
+    # Ensure default markersize is still 20 if no rcparams
+    df = DataFrame(data={"x": [1, 2, 3], "y": [1, 2, 3]})
+    with mpl.rc_context({"lines.markersize": 10}):
+        pandas_default_with_rcparams = df.plot.scatter(
+            x="x", y="y", title="pandas scatter, changed rc marker size"
+        )
+    # Verify that pandas default markersize is still 20 with rc_params
+    assert pandas_default_with_rcparams.collections[0].get_sizes()[0] == 20
+
+
+@pytest.mark.filterwarnings(
+    "ignore:The default of s=20 will be changed:pandas.errors.PandasFutureWarning"
+)
+def test_scatter_markersize_same_default_without_rcparams():
+    # GH 54204
+    # Ensure default markersize is still 20 if no rcparams
+    df = DataFrame(data={"x": [1, 2, 3], "y": [1, 2, 3]})
+
+    pandas_default_with_rcparams = df.plot.scatter(
+        x="x", y="y", title="pandas scatter, changed rc marker size"
+    )
+    # Verify that pandas default markersize is still 20 without rc_params
+    assert pandas_default_with_rcparams.collections[0].get_sizes()[0] == 20