Merge pull request matplotlib#28720 from timhoffm/doc-axhline

DOC: Clarify axhline() uses axes coordinates

Author: timhoffm

galleries/users_explain/artists/transforms_tutorial.py

@@ -22,6 +22,7 @@
 :class:`~matplotlib.figure.Figure` instance, and ``subfigure`` is a
 :class:`~matplotlib.figure.SubFigure` instance.
 
+.. _coordinate-systems:
 
 +----------------+-----------------------------------+-----------------------------+
 |Coordinate      |Description                        |Transformation object        |

lib/matplotlib/axes/_axes.py

@@ -767,20 +767,25 @@ def annotate(self, text, xy, xytext=None, xycoords='data', textcoords=None,
     @_docstring.dedent_interpd
     def axhline(self, y=0, xmin=0, xmax=1, **kwargs):
         """
-        Add a horizontal line across the Axes.
+        Add a horizontal line spanning the whole or fraction of the Axes.
+
+        Note: If you want to set x-limits in data coordinates, use
+        `~.Axes.hlines` instead.
 
         Parameters
         ----------
         y : float, default: 0
-            y position in data coordinates of the horizontal line.
+            y position in :ref:`data coordinates <coordinate-systems>`.
 
         xmin : float, default: 0
-            Should be between 0 and 1, 0 being the far left of the plot, 1 the
-            far right of the plot.
+            The start x-position in :ref:`axes coordinates <coordinate-systems>`.
+            Should be between 0 and 1, 0 being the far left of the plot,
+            1 the far right of the plot.
 
         xmax : float, default: 1
-            Should be between 0 and 1, 0 being the far left of the plot, 1 the
-            far right of the plot.
+            The end x-position in :ref:`axes coordinates <coordinate-systems>`.
+            Should be between 0 and 1, 0 being the far left of the plot,
+            1 the far right of the plot.
 
         Returns
         -------
@@ -836,18 +841,23 @@ def axhline(self, y=0, xmin=0, xmax=1, **kwargs):
     @_docstring.dedent_interpd
     def axvline(self, x=0, ymin=0, ymax=1, **kwargs):
         """
-        Add a vertical line across the Axes.
+        Add a vertical line spanning the whole or fraction of the Axes.
+
+        Note: If you want to set y-limits in data coordinates, use
+        `~.Axes.vlines` instead.
 
         Parameters
         ----------
         x : float, default: 0
-            x position in data coordinates of the vertical line.
+            y position in :ref:`data coordinates <coordinate-systems>`.
 
         ymin : float, default: 0
+            The start y-position in :ref:`axes coordinates <coordinate-systems>`.
             Should be between 0 and 1, 0 being the bottom of the plot, 1 the
             top of the plot.
 
         ymax : float, default: 1
+            The end y-position in :ref:`axes coordinates <coordinate-systems>`.
             Should be between 0 and 1, 0 being the bottom of the plot, 1 the
             top of the plot.