plot_utils: fix setting the size of the plots

Longstanding bug, finally fixed.  Merge set_plot_size() into
pre_plot_setup() in the process as it's the only thing that it does.
diff --git a/tests/test_plot_utils.py b/tests/test_plot_utils.py
index c6a1163..31b7094 100644
--- a/tests/test_plot_utils.py
+++ b/tests/test_plot_utils.py
@@ -7,13 +7,6 @@
 import plot_utils
 
 class TestPlotUtils(unittest.TestCase):
-    def test_set_plot_size(self):
-        """Test that plot_utils.set_plot_size() doesn't bomb"""
-        plot_utils.set_plot_size(None, None)
-        plot_utils.set_plot_size(height=9, width=None)
-        plot_utils.set_plot_size(height=None, width=9)
-        plot_utils.set_plot_size(3, 9)
-
     def test_normalize_title(self):
         """Test normalize_title"""
         self.assertEquals(plot_utils.normalize_title("Foo", ""), "Foo")
@@ -64,6 +57,13 @@
         plot_utils.set_xlim(ax, "default")
         plot_utils.set_xlim(ax, (0, 5))
 
+    def test_pre_plot_setup(self):
+        """Test that plot_utils.pre_plot_setup() doesn't bomb"""
+        plot_utils.pre_plot_setup(None, None)
+        plot_utils.pre_plot_setup(height=9, width=None)
+        plot_utils.pre_plot_setup(height=None, width=9)
+        plot_utils.pre_plot_setup(3, 9)
+
     def test_post_plot_setup(self):
         """Test that post_plot_setup() doesn't bomb"""