Fix fencepost problem with colorbar bins

This makes the numbers on the colourbar labels much rounder if the
range is a multiple of 20.

This comes from the fencepost problem, which describes how you need
n+1 dividers to support n bins. For example, imagine a fence with 5
segments, like so:

    |###|###|###|###|###|

Notice that we have 6 posts (denoted with | characters) for our 5
fence segments. This is the fence post problem.

src/CSET/operators/plot.py

@@ -234,7 +234,7 @@ def _colorbar_map_levels(cube: iris.cube.Cube):
                 vmin, vmax = var_colorbar["min"], var_colorbar["max"]
                 logging.debug("Using min and max for %s colorbar.", varname)
                 # Calculate levels from range.
-                levels = np.linspace(vmin, vmax, 20)
+                levels = np.linspace(vmin, vmax, 21)
                 norm = None
             return cmap, levels, norm
         except KeyError:
@@ -709,7 +709,7 @@ def _plot_and_save_histogram_series(
             vmin = 0
             vmax = 400  # Manually set vmin/vmax to override json derived value.
         else:
-            bins = np.linspace(vmin, vmax, 50)
+            bins = np.linspace(vmin, vmax, 51)
 
         # Reshape cube data into a single array to allow for a single histogram.
         # Otherwise we plot xdim histograms stacked.

tests/operators/test_plots.py

@@ -88,7 +88,7 @@ def test_colorbar_map_levels(cube, tmp_working_dir):
     """Colorbar definition is found for cube."""
     cmap, levels, norm = plot._colorbar_map_levels(cube)
     assert cmap == mpl.colormaps["RdYlBu_r"]
-    assert (levels == np.linspace(223, 323, 20)).all()
+    assert (levels == np.linspace(223, 323, 21)).all()
     assert norm is None
 
 
@@ -108,7 +108,7 @@ def test_colorbar_map_levels_name_fallback(cube, tmp_working_dir):
     cube.standard_name = None
     cmap, levels, norm = plot._colorbar_map_levels(cube)
     assert cmap == mpl.colormaps["RdYlBu_r"]
-    assert (levels == np.linspace(223, 323, 20)).all()
+    assert (levels == np.linspace(223, 323, 21)).all()
     assert norm is None
 
 
@@ -128,7 +128,7 @@ def test_colorbar_map_levels_pressure_level(transect_source_cube, tmp_working_di
     cube_250hPa = transect_source_cube.extract(iris.Constraint(pressure=250))
     cmap, levels, norm = plot._colorbar_map_levels(cube_250hPa)
     assert cmap == mpl.colormaps["RdYlBu_r"]
-    assert (levels == np.linspace(200, 240, 20)).all()
+    assert (levels == np.linspace(200, 240, 21)).all()
     assert norm is None