fixes #217 (#231)

The plot_var functions is altered in two ways:
- Adds an edge if input is used in value of a node that has also a distribution.
- Introduces a red edge if input is used in the value node and the dist node of a variable.

liesel/model/viz.py

@@ -173,18 +173,43 @@ def _draw_edges(graph, axis, pos, is_var):
 
     if is_var:
         dist_edges = []
-        non_dist_edges = []
+        value_edges = []
 
         for edge in edges:
+
+            # find distribution edges
             if edge[1].has_dist:
                 edge_0_output_nodes = set(edge[0].all_output_nodes())
                 edge_0_nodes = edge[0].nodes
                 edge_1_input_nodes = set(edge[1].dist_node.all_input_nodes())
 
                 if bool(edge_0_output_nodes.union(edge_0_nodes) & edge_1_input_nodes):
                     dist_edges.append(edge)
-            else:
-                non_dist_edges.append(edge)
+
+            # find value edges
+            edge_0_output_nodes = set(edge[0].all_output_nodes())
+            edge_0_nodes = edge[0].nodes
+            edge_1_input_nodes = set(edge[1].value_node.all_input_nodes())
+
+            if bool(edge_0_output_nodes.union(edge_0_nodes) & edge_1_input_nodes):
+                value_edges.append(edge)
+
+        edges_in_both = set(dist_edges) & set(value_edges)
+        dist_edges = set(dist_edges) - edges_in_both
+        value_edges = set(value_edges) - edges_in_both
+
+        # assigns value_edges to edges to make it comparible with is_var=False
+        edges = value_edges
+
+        nx.draw_networkx_edges(
+            graph,
+            pos,
+            edgelist=edges_in_both,
+            edge_color="#FF0000",
+            arrows=True,
+            ax=axis,
+            node_size=500,
+        )
 
         nx.draw_networkx_edges(
             graph,
@@ -196,8 +221,6 @@ def _draw_edges(graph, axis, pos, is_var):
             node_size=500,
         )
 
-        edges = non_dist_edges
-
     nx.draw_networkx_edges(
         graph,
         pos,
@@ -247,12 +270,22 @@ def _add_legend(axis):
             [0],
             [0],
             marker=r"$\rightarrow$",
-            color="#aaaaaa",
+            color="#AAAAAA",
             label="Used in distribution",
             markerfacecolor="k",
             markersize=12,
             lw=0,
         ),
+        Line2D(
+            [0],
+            [0],
+            marker=r"$\rightarrow$",
+            color="#FF0000",
+            label="Used in value and distribution",
+            markerfacecolor="k",
+            markersize=12,
+            lw=0,
+        ),
     ]
 
     axis.legend(handles=legend_elements, loc="best")