plot_utils: add the unit to the title of the histogram plot

Xin requested units in the mean calculated in the title of the
histogram.  Pass the unit as a string explicitly to plot_hist() and let
that function add it to the xlabel and the title.

Change-Id: If617d91617dc2cabf0ce43795581a6240ca40e4e
diff --git a/tests/test_plot_utils.py b/tests/test_plot_utils.py
index 21f289a..cd44cca 100644
--- a/tests/test_plot_utils.py
+++ b/tests/test_plot_utils.py
@@ -92,7 +92,7 @@
         data = pd.Series([1, 1, 2, 4])
 
         _, ax = matplotlib.pyplot.subplots()
-        plot_utils.plot_hist(data, ax, "Foo", 20, "numbers", (0, 4), "default")
+        plot_utils.plot_hist(data, ax, "Foo", "m", 20, "numbers", (0, 4), "default")
 
 class TestPlotUtilsNeedTrace(BaseTestThermal):
     def __init__(self, *args, **kwargs):