Fixed bug with priority of rcParams

In version 4.0.0 I added theme="default", which was then going to be applied
every time a new figure was instantiated. Turns out that setting a theme with
`plt.style.context(theme)` is going to disregard user-provided settings with
`plt.rcParams`. This commit sets theme="", which would still use Matplotlib
default theme, but it will be loaded through rcParams. So any user-provided
settings to rcParams will be applied to the figure, provided that theme="".

Author: Davide-sd

spb/backends/matplotlib/matplotlib.py

@@ -154,7 +154,7 @@ def __init__(self, *args, **kwargs):
             "use_latex", cfg["matplotlib"]["use_latex"])
         kwargs.setdefault("grid", cfg["matplotlib"]["grid"])
         kwargs.setdefault("minor_grid", cfg["matplotlib"]["show_minor_grid"])
-        kwargs.setdefault("theme", "default")
+        kwargs.setdefault("theme", "")
         super().__init__(*args, **kwargs)
 
         # set labels
@@ -634,10 +634,21 @@ def draw(self):
         # create the figure from scratch every time, otherwise if the plot was
         # previously shown, it would not be possible to show it again. This
         # behaviour is specific to Matplotlib
-        with self.plt.style.context(self.theme):
+        def func():
             self._create_figure()
             self._process_renderers()
 
+        # NOTE: when applying a theme, plt.rcParams is not considered.
+        # So, any user settings to plt.rcParams will be disregarded.
+        # The following if/else assures that if no theme is provided, then
+        # plt.rcParams are going to be considered.
+
+        if self.theme:
+            with self.plt.style.context(self.theme):
+                func()
+        else:
+            func()
+
     process_series = draw
 
     def show(self, **kwargs):