Add support for suptitle() in tight_layout().

The padding between the suptitle and the axes is the same as around all
axes, which is consistent with the behavior of constrained_layout.

doc/users/next_whats_new/2020-04-22-suptitle-tl.rst

@@ -0,0 +1,2 @@
+tight_layout now supports suptitle
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

lib/matplotlib/tests/test_tightlayout.py

@@ -308,3 +308,11 @@ def test_collapsed():
     # test that passing a rect doesn't crash...
     with pytest.warns(UserWarning):
         plt.tight_layout(rect=[0, 0, 0.8, 0.8])
+
+
+def test_suptitle():
+    fig, ax = plt.subplots(tight_layout=True)
+    st = fig.suptitle("foo")
+    t = ax.set_title("bar")
+    fig.canvas.draw()
+    assert st.get_window_extent().y0 > t.get_window_extent().y1

lib/matplotlib/tight_layout.py

@@ -107,9 +107,15 @@ def auto_adjust_subplotpars(
     if not margin_top:
         margin_top = (max(vspaces[0, :].max(), 0)
                       + pad_inches / fig_height_inch)
+        suptitle = fig._suptitle
+        if suptitle and suptitle.get_in_layout():
+            rel_suptitle_height = fig.transFigure.inverted().transform_bbox(
+                suptitle.get_window_extent(renderer)).height
+            margin_top += rel_suptitle_height + pad_inches / fig_height_inch
     if not margin_bottom:
         margin_bottom = (max(vspaces[-1, :].max(), 0)
                          + pad_inches / fig_height_inch)
+
     if margin_left + margin_right >= 1:
         cbook._warn_external('Tight layout not applied. The left and right '
                              'margins cannot be made large enough to '