Merge remote-tracking branch 'upstream/v1.2.x' into HEAD

Conflicts:
	src/_backend_agg.cpp

Author: mdboom

lib/matplotlib/backend_bases.py

@@ -898,6 +898,10 @@ def set_dashes(self, dash_offset, dash_list):
             ``(None, None)`` specifies a solid line
 
         """
+        if dash_list is not None:
+            dl = np.asarray(dash_list)
+            if np.any(dl <= 0.0):
+                raise ValueError("All values in the dash list must be positive")
         self._dashes = dash_offset, dash_list
 
     def set_foreground(self, fg, isRGBA=False):