plot_utils: use the axis' set_xlabel to set the xlabel
diff --git a/cr2/plot_utils.py b/cr2/plot_utils.py
index a1a94de..fe47833 100644
--- a/cr2/plot_utils.py
+++ b/cr2/plot_utils.py
@@ -88,7 +88,7 @@
     """
 
     if xlabel is not None:
-        plt.xlabel(xlabel)
+        ax.set_xlabel(xlabel)
 
     if title:
         ax.set_title(title)