matplotlib · QuLogic · Dec 15, 2020 · Oct 3, 2020 · Oct 3, 2020 · Oct 3, 2020
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -876,6 +876,55 @@ def _to_points(xy1, xy2, slope):
         self._request_autoscale_view()
         return line
 
+    def axline_transaxes(self, xy1, slope, **kwargs):
+        """
+        Add an infinitely long straight line.
+
+        The line is defined by one point in axes coordinates *xy1* and a
+        *slope* in data coordinates.
+
+        This should only be used with linear scales; the *slope* has no clear
+        meaning for all other scales, and thus the behavior is undefined.
+
+        Parameters
+        ----------
+        xy1 : (float, float)
+            Point for the line to pass through, in axes coordinates.
+        slope : float
+            The slope of the line, in data coordinates.
+
+        Returns
+        -------
+        `.Line2D`
+
+        Other Parameters
+        ----------------
+        **kwargs
+            Valid kwargs are `.Line2D` properties, with the exception of
+            'transform':
+
+            %(_Line2D_docstr)s
+
+        See Also
+        --------
+        axhline : for horizontal lines
+        axvline : for vertical lines
+
+        Examples
+        --------
+        Draw a thick red line passing through (0, 0)
+        (the lower left corner of the viewport) with slope 1::
+
+            >>> axline_transaxes((0, 0), 1, linewidth=4, color='r')
+        """
+        if "transform" in kwargs:
+            raise TypeError("'transform' is not allowed as a kwarg; "
+                            "axline_transaxes generates its own transform")
+
+        line = mlines._AxLineTransAxes(xy1, slope, **kwargs)
+        self.add_line(line)
+        return line
+
     @docstring.dedent_interpd
     def axhspan(self, ymin, ymax, xmin=0, xmax=1, **kwargs):
         """

diff --git a/lib/matplotlib/lines.py b/lib/matplotlib/lines.py
@@ -1440,6 +1440,34 @@ def draw(self, renderer):
         super().draw(renderer)
 
 
+class _AxLineTransAxes(_AxLine):
+    """
+    A helper class that implements `~.Axes.axline_transaxes`, by recomputing
+    the artist transform at draw time.
+    """
+
+    def __init__(self, xy1, slope, **kwargs):
+        super().__init__([0, 1], [0, 1], **kwargs)
+        self._slope = slope
+        self._xy1 = xy1
+
+    def get_transform(self):
+        ax = self.axes
+        (vxlo, vylo), (vxhi, vyhi) = ax.transScale.transform(ax.viewLim)
+        x, y = (ax.transAxes + ax.transData.inverted()).transform(self._xy1)
+
+        # find intersections with view limits in either direction,
+        # and draw between the middle two points.
+        _, start, stop, _ = sorted([
+            (vxlo, y + (vxlo - x) * self._slope),
+            (vxhi, y + (vxhi - x) * self._slope),
+            (x + (vylo - y) / self._slope, vylo),
+            (x + (vyhi - y) / self._slope, vyhi),
+        ])
+        return (BboxTransformTo(Bbox([start, stop]))
+                + ax.transLimits + ax.transAxes)
+
+
 class VertexSelector:
     """
     Manage the callbacks to maintain a list of selected vertices for

diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -4072,6 +4072,19 @@ def test_axline(fig_test, fig_ref):
     ax.axvline(-0.5, color='C5')
 
 
+@check_figures_equal()
+def test_axline_transform(fig_test, fig_ref):
+    ax = fig_test.subplots()
+    ax.set(xlim=(-1, 1), ylim=(-1, 1))
+    ax.axline_transaxes((0, 0), slope=1)
+    ax.axline_transaxes((1, 0.5), slope=1, color='C1')
+
+    ax = fig_ref.subplots()
+    ax.set(xlim=(-1, 1), ylim=(-1, 1))
+    ax.plot([-1, 1], [-1, 1])
+    ax.plot([0, 1], [-1, 0], color='C1')
+
+
 def test_axline_args():
     """Exactly one of *xy2* and *slope* must be specified."""
     fig, ax = plt.subplots()