Move matplotlib import

Move a matplotlib import into _draw_graph to avoid one-off dependency issue.

Author: Corey Ostrove

pygsti/layouts/prefixtable.py

@@ -12,7 +12,6 @@
 
 import collections as _collections
 import networkx as _nx
-import matplotlib.pyplot as plt
 from math import ceil
 from pygsti.baseobjs import Label as _Label
 from pygsti.circuits.circuit import SeparatePOVMCircuit as _SeparatePOVMCircuit
@@ -1208,6 +1207,14 @@ def _draw_graph(G, node_label_key='label', edge_label_key='promotion_cost', figu
         An optional size specifier passed into the matplotlib figure
         constructor to set the plot size.
     """
+    try:
+        import matplotlib.pyplot as plt
+    except ImportError as e:
+        msg = 'The function `_draw_graph` requires the installation of matplotlib.'\
+              +'Please ensure this is properly installed and try again.'
+        print(msg)
+        raise e
+
     plt.figure(figsize=figure_size)
     pos = _nx.nx_agraph.graphviz_layout(G, prog="dot", args="-Granksep=5 -Gnodesep=10")
     labels = _nx.get_node_attributes(G, node_label_key)