Fix order of legend, so plot doesn't overlap (#92)

* changed default order of plots to 4

* changed default order of barbs to 4.1

* added comments

tephi/isopleths.py

@@ -368,7 +368,9 @@ def plot(self, barbs, **kwargs):
 
         """
         self._gutter = kwargs.pop("gutter", _BARB_GUTTER)
-        self._kwargs = dict(length=7, zorder=10)
+        # zorder of 4.1 is higher than all MPL defaults, excluding legend. Also
+        # higher than tephi default for plot-lines.
+        self._kwargs = dict(length=7, zorder=4.1)
         self._kwargs.update(kwargs)
         self._custom_kwargs = dict(
             color=None, linewidth=1.5, zorder=self._kwargs["zorder"]
@@ -444,8 +446,9 @@ def plot(self, **kwargs):
         if self.line is not None and self.line in self.axes.lines:
             self.axes.lines.remove(self.line)
 
+        # zorder of 4 is higher than all MPL defaults, excluding legend.
         if "zorder" not in kwargs:
-            kwargs["zorder"] = 10
+            kwargs["zorder"] = 4
 
         (self.line,) = self.axes.plot(
             self.temperature, self.theta, transform=self._transform, **kwargs