plot_utils: add a "count" y-label to the histogram plot

Xin was confused about the histogram plot.  It doesn't look like an
histogram.  Add a Y-label explaining that it is a count.  Add support
for y-labels to post_plot_setup() accordingly.

Change-Id: I1be871e89dead9f35996bb89df68e088521c0349
diff --git a/cr2/plot_utils.py b/cr2/plot_utils.py
index 3ae88de..fc01329 100644
--- a/cr2/plot_utils.py
+++ b/cr2/plot_utils.py
@@ -92,8 +92,9 @@
 
     return axis
 
-def post_plot_setup(ax, title="", xlabel=None, xlim="default", ylim="range"):
-    """Set xlabel, title, xlim adn ylim of the plot
+def post_plot_setup(ax, title="", xlabel=None, ylabel=None, xlim="default",
+                    ylim="range"):
+    """Set xlabel, ylabel title, xlim and ylim of the plot
 
     This has to be called after calls to .plot().  The default ylim is
     to extend it by 10% because matplotlib default makes it hard
@@ -104,6 +105,9 @@
     if xlabel is not None:
         ax.set_xlabel(xlabel)
 
+    if ylabel is not None:
+        ax.set_ylabel(ylabel)
+
     if title:
         ax.set_title(title)
 
@@ -144,7 +148,8 @@
     title += " (mean = {:.2f}, std = {:.2f})".format(mean, std)
 
     data.hist(ax=ax, bins=bins)
-    post_plot_setup(ax, title=title, xlabel=xlabel, xlim=xlim, ylim=ylim)
+    post_plot_setup(ax, title=title, xlabel=xlabel, ylabel="count", xlim=xlim,
+                    ylim=ylim)
 
 def plot_load(runs, map_label, width=None, height=None):
     """Make a multiplot of all the loads"""