Choppy video of a mobile network

I am creating a video of a mobile network (graph). For this case, they are following a simple mobility model: everyone moves right a small amount at every time step. Each image is written to file and loaded back in and written to video (via OpenCV).

As far as I can tell the layout is turned off, but their movement is strangely choppy, plus their y-position changes a bit too. The resulting video of 1,000 steps is attached.

The drawing code for one frame is as follows:

gt.graph_draw(g,
              g.vp.locs, # i thought this enforces node position & turns off layout
              vertex_size = 10,
              vertex_pen_width = 0.8, # default is 0.8
              vertex_color = color['black'], # vertex stroke color
              vertex_fill_color = color['red'],
              edge_color = color['white'],
              edge_pen_width = 0.8, # default is 1
              vertex_anchor = 0,
              bg_color = color['black'],
              output_size = [iw, ih],
              adjust_aspect = False,
              output = img_out_path)
img = cm.imread(img_out_path) # this is my image reading function

graph.mkv

In addition, if I try to set fit_view = False to test if that fixes things I get the following error:

File "/usr/lib/python3/dist-packages/graph_tool/draw/cairo_draw.py", line 1146, in graph_draw
    x, y, zoom = x, y, 1
UnboundLocalError: local variable 'x' referenced before assignment