Close figures after saving, and refactor plotting code

Refactors the plotting code to use the explicit (object-oriented)
matplotlib API. Figures are explicitly closed, preventing memory
leaking. Also added a super-title to the postage stamp plots, so
the diagnostic title is there too.

Fixes #481

src/CSET/operators/plot.py

@@ -16,6 +16,7 @@
 
 import fcntl
 import importlib.resources
+import itertools
 import json
 import logging
 import math
@@ -138,32 +139,33 @@ def _plot_and_save_contour_plot(
     """
     # Setup plot details, size, resolution, etc.
     fig = plt.figure(figsize=(15, 15), facecolor="w", edgecolor="k")
-
-    cmap = mpl.colormaps["viridis"]
+    axes = fig.subplots()
 
     # Filled contour plot of the field.
-    iplt.contourf(cube, cmap=cmap)
-    axes = fig.gca()
+    cmap = mpl.colormaps["viridis"]
+    iplt.contourf(cube, cmap=cmap, axes=axes)
 
     # Add coastlines.
     axes.coastlines(resolution="10m")
 
     # Add title.
-    plt.title(title, fontsize=16)
+    axes.set_title(title, fontsize=16)
 
     # Add colour bar.
-    cbar = plt.colorbar()
+    cbar = fig.colorbar()
     cbar.set_label(label=f"{cube.name()} ({cube.units})", size=20)
 
     # Save plot.
     fig.savefig(filename, bbox_inches="tight", dpi=150)
     logging.info("Saved contour plot to %s", filename)
+    plt.close(fig)
 
 
 def _plot_and_save_postage_stamp_contour_plot(
     cube: iris.cube.Cube,
     filename: str,
     stamp_coordinate: str,
+    title: str,
     **kwargs,
 ):
     """Plot postage stamp contour plots from an ensemble.
@@ -185,24 +187,29 @@ def _plot_and_save_postage_stamp_contour_plot(
     # Use the smallest square grid that will fit the members.
     grid_size = int(math.ceil(math.sqrt(len(cube.coord(stamp_coordinate).points))))
 
-    plt.figure(figsize=(10, 10))
-    # Make a subplot for each member.
-    subplot = 1
-    for member in cube.slices_over(stamp_coordinate):
-        plt.subplot(grid_size, grid_size, subplot)
-        plot = iplt.contourf(member)
-        plt.title(f"Member #{member.coord(stamp_coordinate).points[0]}")
-        plt.axis("off")
-        plt.gca().coastlines()
-        subplot += 1
+    fig = plt.figure(figsize=(10, 10))
+    # Make a subplot for each member. This returns a 2D array, which we turn
+    # into a 1D iterable.
+    axes = itertools.chain.from_iterable(
+        fig.subplots(grid_size, grid_size, squeeze=False)
+    )
+    for ax, member in zip(axes, cube.slices_over(stamp_coordinate)):
+        plot = iplt.contourf(member, axes=ax)
+        ax.set_title(f"Member #{member.coord(stamp_coordinate).points[0]}")
+        ax.set_axis_off()
+        ax.coastlines()
 
     # Put the shared colorbar in its own axes.
-    colorbar_axes = plt.gcf().add_axes([0.15, 0.07, 0.7, 0.03])
-    colorbar = plt.colorbar(plot, colorbar_axes, orientation="horizontal")
+    colorbar_axes = fig.add_axes([0.15, 0.07, 0.7, 0.03])
+    colorbar = fig.colorbar(plot, colorbar_axes, orientation="horizontal")
     colorbar.set_label(f"{cube.name()} / {cube.units}")
 
-    plt.savefig(filename, bbox_inches="tight", dpi=150)
+    # Overall figure title.
+    fig.suptitle(title)
+
+    fig.savefig(filename, bbox_inches="tight", dpi=150)
     logging.info("Saved contour postage stamp plot to %s", filename)
+    plt.close(fig)
 
 
 def _plot_and_save_line_series(
@@ -222,10 +229,10 @@ def _plot_and_save_line_series(
         Plot title.
     """
     fig = plt.figure(figsize=(8, 8), facecolor="w", edgecolor="k")
-    iplt.plot(coord, cube, "o-")
+    ax = fig.subplots()
+    iplt.plot(coord, cube, "o-", axes=ax)
 
     # Add some labels and tweak the style.
-    ax = fig.gca()
     ax.set(
         xlabel=f"{coord.name()} / {coord.units}",
         ylabel=f"{cube.name()} / {cube.units}",
@@ -238,6 +245,7 @@ def _plot_and_save_line_series(
     # Save plot.
     fig.savefig(filename, bbox_inches="tight", dpi=150)
     logging.info("Saved line plot to %s", filename)
+    plt.close(fig)
 
 
 ####################
@@ -387,7 +395,7 @@ def postage_stamp_contour_plot(
     except iris.exceptions.CoordinateNotFoundError as err:
         raise ValueError(f"Cube must have a {coordinate} coordinate.") from err
 
-    _plot_and_save_postage_stamp_contour_plot(cube, filename, coordinate)
+    _plot_and_save_postage_stamp_contour_plot(cube, filename, coordinate, title="")
     _make_plot_html_page([filename])
     return cube