More flexible subplot lettering (#94)

* More flexible subplot lettering

* Fix linter

xarrayutils/plotting.py

@@ -110,7 +110,7 @@ def axis_arrow(ax, x_loc, text, arrowprops={}, **kwargs):
  )
 
 
-def letter_subplots(axes, start_idx=0, box_color=None, **kwargs):
+def letter_subplots(axes, start_idx=0, box_color=None, labels=None, **kwargs):
  """Adds panel letters in boxes to each element of `axes` in the
  upper left corner.
 
@@ -122,12 +122,16 @@ def letter_subplots(axes, start_idx=0, box_color=None, **kwargs):
  Starting index in the alphabet (e.g. 0 is 'a').
  box_color : type
  Color of the box behind each letter (the default is None).
+ labels: list
+ List of strings used as labels (if None (default), uses lowercase alphabet followed by uppercase alphabet)
  **kwargs : type
  kwargs passed to matplotlib.axis.text
 
  """
+ if labels is None:
+ labels = list(string.ascii_letters)
 
- for ax, letter in zip(axes.flat, list(string.ascii_lowercase)[start_idx:]):
+ for ax, letter in zip(axes.flat, labels[start_idx:]):
  t = ax.text(
  0.1,
  0.85,