matplotlib · shawnchenx6 · Mar 3, 2020 · Mar 3, 2020 · Apr 7, 2020 · Apr 7, 2020
diff --git a/lib/matplotlib/colorbar.py b/lib/matplotlib/colorbar.py
@@ -1569,10 +1569,6 @@ def make_axes_gridspec(parent, *, fraction=0.15, shrink=1.0, aspect=20, **kw):

    x1 = 1 - fraction

-    # for shrinking
-    pad_s = (1 - shrink) * 0.5
-    wh_ratios = [pad_s, shrink, pad_s]
-
    # we need to none the tree of layoutboxes because
    # constrained_layout can't remove and replace the tree
    # hierarchy w/o a seg fault.
@@ -1582,6 +1578,12 @@ def make_axes_gridspec(parent, *, fraction=0.15, shrink=1.0, aspect=20, **kw):
    if orientation == 'vertical':
        pad = kw.pop('pad', 0.05)
        wh_space = 2 * pad / (1 - pad)
+        anchor = kw.pop('anchor', (0.0, 0.5))
+        panchor = kw.pop('panchor', (1.0, 0.5))
+
+        # for shrinking
+        wh_ratios = [(1-anchor[1])*(1-shrink), shrink, anchor[1]*(1-shrink)]
+
        gs = gs_from_subplotspec(1, 2,
                                 subplot_spec=parent.get_subplotspec(),
                                 wspace=wh_space,
@@ -1590,11 +1592,15 @@ def make_axes_gridspec(parent, *, fraction=0.15, shrink=1.0, aspect=20, **kw):
                                  subplot_spec=gs[1],
                                  hspace=0.,
                                  height_ratios=wh_ratios)
-        anchor = (0.0, 0.5)
-        panchor = (1.0, 0.5)
    else:
        pad = kw.pop('pad', 0.15)
        wh_space = 2 * pad / (1 - pad)
+        anchor = kw.pop('anchor', (0.5, 1.0))
+        panchor = kw.pop('panchor', (0.5, 0.0))
+
+        # for shrinking
+        wh_ratios = [anchor[0]*(1-shrink), shrink, (1-anchor[0])*(1-shrink)]
+
        gs = gs_from_subplotspec(2, 1,
                                 subplot_spec=parent.get_subplotspec(),
                                 hspace=wh_space,
@@ -1604,8 +1610,6 @@ def make_axes_gridspec(parent, *, fraction=0.15, shrink=1.0, aspect=20, **kw):
                                  wspace=0.,
                                  width_ratios=wh_ratios)
        aspect = 1 / aspect
-        anchor = (0.5, 1.0)
-        panchor = (0.5, 0.0)

    parent.set_subplotspec(gs[0])
    parent.update_params()

diff --git a/lib/matplotlib/tests/test_colorbar.py b/lib/matplotlib/tests/test_colorbar.py
@@ -613,3 +613,51 @@ def test_colorbar_int(clim):
    im = ax.imshow([[*map(np.int16, clim)]])
    fig.colorbar(im)
    assert (im.norm.vmin, im.norm.vmax) == clim
+
+
+def test_anchored_cbar_position_using_specgrid():
+    data = np.arange(1200).reshape(30, 40)
+    levels = [0, 200, 400, 600, 800, 1000, 1200]
+    shrink = 0.5
+    anchor_y = 0.3
+    # vertival
+    fig, ax = plt.subplots()
+    cs = ax.contourf(data, levels=levels)
+    cbar = plt.colorbar(
+            cs, ax=ax, use_gridspec=True,
+            orientation='vertical', anchor=(1, anchor_y), shrink=shrink)
+
+    # y1: the top of ax, y0: the bottom of ax, p0: the y postion of anchor
+    # cy1 : the top of colorbar ax, cy0: the bottom of colorbar ax
+    y1 = ax.get_position().y1
+    y0 = ax.get_position().y0
+    p0 = (y1 - y0) * anchor_y + y0
+    cy1 = cbar.ax.get_position().y1
+    cy0 = cbar.ax.get_position().y0
+
+    assert np.isclose(
+            [cy1, cy0],
+            [y1 * shrink + (1 - shrink) * p0, p0 * (1 - shrink) + y0 * shrink]
+            ).all()
+
+    # horizontal
+    shrink = 0.5
+    anchor_x = 0.3
+    fig, ax = plt.subplots()
+    cs = ax.contourf(data, levels=levels)
+    cbar = plt.colorbar(
+            cs, ax=ax, use_gridspec=True,
+            orientation='horizontal', anchor=(anchor_x, 1), shrink=shrink)
+
+    # x1: the right of ax, x0: the left of ax, p0: the x postion of anchor
+    # cx1 : the right of colorbar ax, cx0: the left of colorbar ax
+    x1 = ax.get_position().x1
+    x0 = ax.get_position().x0
+    p0 = (x1 - x0) * anchor_x + x0
+    cx1 = cbar.ax.get_position().x1
+    cx0 = cbar.ax.get_position().x0
+
+    assert np.isclose(
+            [cx1, cx0],
+            [x1 * shrink + (1 - shrink) * p0, p0 * (1 - shrink) + x0 * shrink]
+            ).all()