Cleanly handle case when show_nr_indiv is large

We won't try to render more graphs than we have data for.

causing/graph.py

@@ -252,7 +252,7 @@ def make_graph(filename, x_weights_idmat_nodeff, y_weights_idmat_nodeff, **kwarg
     mediation_indiv_graphs = []
     print()
 
-    for i in range(show_nr_indiv):
+    for i in range(min(show_nr_indiv, len(graph_json["mx_indivs"]))):
         # compute color base for each individual separately
         # using _indiv quantities based on _theo quantities times absolute deviation from median
         print("Generate graphs for individual {:5}".format(i))