[FIXED] Add external margins with constrained layout?

Issue

When generating a figure to save to a pdf file, I'd like to adjust the positioning of the figure relative to the edges of the page, for example to add an inch margin along all sides. As far as I can tell, the solutions to do this (for example, in this question) either:

1. don't work with constrained_layout mode -- applying plt.subplots_adjust() after creating the figure but prior to fig.savefig() messes up the constrained layout
2. don't actually quantitatively adjust the positioning of the figure -- adding bbox_inches="tight" or pad=-1 don't seem to do anything meaningful

Is there a straightforward way to adjust external margins of a constrained layout figure?

For example:

fig = plt.figure(constrained_layout=True, figsize=(11, 8.5))

page_grid = gridspec.GridSpec(nrows=2, ncols=1, figure=fig)

# this doesn't appear to do anything with constrained_layout=True
page_grid.update(left=0.2, right=0.8, bottom=0.2, top=0.8)

top_row_grid = gridspec.GridSpecFromSubplotSpec(1, 3, subplot_spec=page_grid[0])
for i in range(3):
    ax = fig.add_subplot(top_row_grid[:, i], aspect="equal")

n_bottom_row_plots = 10
qc_grid = gridspec.GridSpecFromSubplotSpec(1, n_bottom_row_plots, subplot_spec=page_grid[1])
for i, metric in enumerate(range(n_bottom_row_plots)):
    ax = fig.add_subplot(qc_grid[:, i])
    plt.plot(np.arange(5), np.arange(5))

fig.suptitle("my big label", fontweight="bold", fontsize="x-large", y=0.9)

# this ruins the constrained layout
# plt.subplots_adjust(left=0.2,right=0.8, bottom=0.2, top=0.8)

fig.savefig("temp.png", facecolor="coral")

Yields the following (I'd like to see more coral around the edges!):

Solution

You can set the rectangle that the layout engine operates within. See the rect parameter for each engine at https://matplotlib.org/stable/api/layout_engine_api.html.

It's unfortunately not a very friendly part of the API, especially because TightLayoutEngine and ConstrainedLayoutEngine have different semantics for rect: TightLayoutEngine uses rect = (left, bottom, right, top) and ConstrainedLayoutEngine uses rect = (left, bottom, width, height).

def set_margins(fig, margins):
    """Set figure margins as [left, right, top, bottom] in inches
    from the edges of the figure."""
    left,right,top,bottom = margins
    width, height = fig.get_size_inches()
    #convert to figure coordinates:
    left, right = left/width, 1-right/width
    bottom, top = bottom/height, 1-top/height
    #get the layout engine and convert to its desired format
    engine = fig.get_layout_engine()
    if isinstance(engine, matplotlib.layout_engine.TightLayoutEngine):
        rect = (left, bottom, right, top)
    elif isinstance(engine, matplotlib.layout_engine.ConstrainedLayoutEngine):
        rect = (left, bottom, right-left, top-bottom)
    else:
        raise RuntimeError('Cannot adjust margins of unsupported layout engine')
    #set and recompute the layout
    engine.set(rect=rect)
    engine.execute(fig)

With your example:

fig = plt.figure(constrained_layout=True, figsize=(11, 8.5))

page_grid = gridspec.GridSpec(nrows=2, ncols=1, figure=fig)

#your margins were [0.2, 0.8, 0.2, 0.8] in figure coordinates
#which are 0.2*11 and 0.2*8.5 in inches from the edge
set_margins(fig,[0.2*11, 0.2*11, 0.2*8.5, 0.2*8.5])

top_row_grid = gridspec.GridSpecFromSubplotSpec(1, 3, subplot_spec=page_grid[0])
for i in range(3):
    ax = fig.add_subplot(top_row_grid[:, i], aspect="equal")

n_bottom_row_plots = 10
qc_grid = gridspec.GridSpecFromSubplotSpec(1, n_bottom_row_plots, subplot_spec=page_grid[1])
for i, metric in enumerate(range(n_bottom_row_plots)):
    ax = fig.add_subplot(qc_grid[:, i])
    plt.plot(np.arange(5), np.arange(5))

fig.suptitle("my big label", fontweight="bold", fontsize="x-large", y=0.9)

fig.savefig("temp.png", facecolor="coral")

Note: fig.suptitle text is apparently not handled by the layout engine, so it doesn't move.

Answered By - Samuel Powell