Fix axline for slopes <= 1E-8. Closes matplotlib#28386 (matplotlib#28881)

Hello, this PR closes matplotlib#28386 by replacing `if np.isclose(slope, 0)` with `if slope == 0` in `lines.py`, allowing for better resolution of small slopes with `axline`s. Additionally, I have added the `test_line_slope` function in `test_lines.py` to ensure proper testing of this functionality.

Author: kyracho

lib/matplotlib/lines.py

@@ -1521,7 +1521,7 @@ def get_transform(self):
         (vxlo, vylo), (vxhi, vyhi) = ax.transScale.transform(ax.viewLim)
         # General case: find intersections with view limits in either
         # direction, and draw between the middle two points.
-        if np.isclose(slope, 0):
+        if slope == 0:
             start = vxlo, y1
             stop = vxhi, y1
         elif np.isinf(slope):

lib/matplotlib/tests/test_lines.py

@@ -436,3 +436,14 @@ def test_axline_setters():
     with pytest.raises(ValueError,
                        match="Cannot set a 'slope' value while 'xy2' is set"):
         line2.set_slope(3)
+
+
+def test_axline_small_slope():
+    """Test that small slopes are not coerced to zero in the transform."""
+    line = plt.axline((0, 0), slope=1e-14)
+    p1 = line.get_transform().transform_point((0, 0))
+    p2 = line.get_transform().transform_point((1, 1))
+    # y-values must be slightly different
+    dy = p2[1] - p1[1]
+    assert dy > 0
+    assert dy < 4e-12