Use Context.save(), Context.restore() stack

lib/xmet/skew_t.py

@@ -104,6 +104,8 @@ class SkewTGraph():
                      y: float,
                      samples: Iterable[SoundingSample],
                      fn: Callable):
+        cr.save()
+
         first = True
 
         for sample in samples:
@@ -129,6 +131,7 @@ class SkewTGraph():
                 cr.line_to(x + sx, y + sy)
 
         cr.stroke()
+        cr.restore()
 
     def draw(self,
              cr: cairo.Context,