Model your feedback loops with FreeStyler.
Drag the node from the right and drop it onto the white left site. If you click on the variable you can change the text. To change the size click on a node and small black quadrangles appear. Drag the view to the desired size using your mouse.
An edge is drawn by dragging the mouse with a pressed left button from the start node to the end node. You can choose between two edges:
1. Generate +/- edge
Click on the green circled + to change to -.
2. Generate graph edge
Click on the blue question mark and you can change the relation of the connected variables.
3. Delete edge
By choosing the function "delete edge" you can delete an edge by dragging the mouse with a pressed left button between the nodes.