Fix empty plot with drawstyle="steps"

lib/matplotlib/cbook/__init__.py

@@ -2167,7 +2167,7 @@ def pts_to_prestep(x, *args):
     Parameters
     ----------
     x : array
-        The x location of the steps.
+        The x location of the steps. May be empty.
 
     y1, ..., yp : array
         y arrays to be turned into steps; all must be the same length as ``x``.
@@ -2177,13 +2177,14 @@ def pts_to_prestep(x, *args):
     out : array
         The x and y values converted to steps in the same order as the input;
         can be unpacked as ``x_out, y1_out, ..., yp_out``.  If the input is
-        length ``N``, each of these arrays will be length ``2N + 1``.
+        length ``N``, each of these arrays will be length ``2N + 1``. For
+        ``N=0``, the length will be 0.
 
     Examples
     --------
     >> x_s, y1_s, y2_s = pts_to_prestep(x, y1, y2)
     """
-    steps = np.zeros((1 + len(args), 2 * len(x) - 1))
+    steps = np.zeros((1 + len(args), max(2 * len(x) - 1, 0)))
     # In all `pts_to_*step` functions, only assign *once* using `x` and `args`,
     # as converting to an array may be expensive.
     steps[0, 0::2] = x
@@ -2204,7 +2205,7 @@ def pts_to_poststep(x, *args):
     Parameters
     ----------
     x : array
-        The x location of the steps.
+        The x location of the steps. May be empty.
 
     y1, ..., yp : array
         y arrays to be turned into steps; all must be the same length as ``x``.
@@ -2214,13 +2215,14 @@ def pts_to_poststep(x, *args):
     out : array
         The x and y values converted to steps in the same order as the input;
         can be unpacked as ``x_out, y1_out, ..., yp_out``.  If the input is
-        length ``N``, each of these arrays will be length ``2N + 1``.
+        length ``N``, each of these arrays will be length ``2N + 1``. For
+        ``N=0``, the length will be 0.
 
     Examples
     --------
     >> x_s, y1_s, y2_s = pts_to_poststep(x, y1, y2)
     """
-    steps = np.zeros((1 + len(args), 2 * len(x) - 1))
+    steps = np.zeros((1 + len(args), max(2 * len(x) - 1, 0)))
     steps[0, 0::2] = x
     steps[0, 1::2] = steps[0, 2::2]
     steps[1:, 0::2] = args
@@ -2239,7 +2241,7 @@ def pts_to_midstep(x, *args):
     Parameters
     ----------
     x : array
-        The x location of the steps.
+        The x location of the steps. May be empty.
 
     y1, ..., yp : array
         y arrays to be turned into steps; all must be the same length as ``x``.
@@ -2258,7 +2260,8 @@ def pts_to_midstep(x, *args):
     steps = np.zeros((1 + len(args), 2 * len(x)))
     x = np.asanyarray(x)
     steps[0, 1:-1:2] = steps[0, 2::2] = (x[:-1] + x[1:]) / 2
-    steps[0, 0], steps[0, -1] = x[0], x[-1]
+    steps[0, :1] = x[:1]  # Also works for zero-sized input.
+    steps[0, -1:] = x[-1:]
     steps[1:, 0::2] = args
     steps[1:, 1::2] = steps[1:, 0::2]
     return steps

lib/matplotlib/tests/test_cbook.py

@@ -392,6 +392,11 @@ def test_to_prestep():
     assert_array_equal(y1_target, y1s)
 
 
+def test_to_prestep_empty():
+    steps = cbook.pts_to_prestep([], [])
+    assert steps.shape == (2, 0)
+
+
 def test_to_poststep():
     x = np.arange(4)
     y1 = np.arange(4)
@@ -412,6 +417,11 @@ def test_to_poststep():
     assert_array_equal(y1_target, y1s)
 
 
+def test_to_poststep_empty():
+    steps = cbook.pts_to_poststep([], [])
+    assert steps.shape == (2, 0)
+
+
 def test_to_midstep():
     x = np.arange(4)
     y1 = np.arange(4)
@@ -432,6 +442,11 @@ def test_to_midstep():
     assert_array_equal(y1_target, y1s)
 
 
+def test_to_midstep_empty():
+    steps = cbook.pts_to_midstep([], [])
+    assert steps.shape == (2, 0)
+
+
 @pytest.mark.parametrize(
     "args",
     [(np.arange(12).reshape(3, 4), 'a'),