plot_utils: use ax.set_title() instead of plt.title()

It lets you set title for each subplot
diff --git a/cr2/plot_utils.py b/cr2/plot_utils.py
index cfd0a93..1d7aed1 100644
--- a/cr2/plot_utils.py
+++ b/cr2/plot_utils.py
@@ -85,7 +85,7 @@
         plt.xlabel(xlabel)
 
     if title:
-        plt.title(title)
+        ax.set_title(title)
 
     set_ylim(ax, ylim)
     set_xlim(ax, xlim)