plot_utils: the height and width of pre_plot_setup() are the ones of the row of plots

This worked ok until pre_plot_setup() learnt to plot multiple rows of
plots.  With this patch, this works again.
diff --git a/cr2/plot_utils.py b/cr2/plot_utils.py
index ae1e750..933e99f 100644
--- a/cr2/plot_utils.py
+++ b/cr2/plot_utils.py
@@ -53,8 +53,9 @@
 def pre_plot_setup(width=None, height=None, ncols=1, nrows=1):
     """initialize a figure
 
-    width and height are numbers.  This function should be called
-    before any calls to plot()
+    width and height are the height and width of each row of plots.
+    For 1x1 plots, that's the height and width of the plot.  This
+    function should be called before any calls to plot()
 
     """
 
@@ -68,6 +69,8 @@
         if width is None:
             width = height * GOLDEN_RATIO
 
+    height *= nrows
+
     _, axis = plt.subplots(ncols=ncols, nrows=nrows, figsize=(width, height))
 
     # Needed for multirow blots to not overlap with each other