States not involved in transitions are missing from the plot() method

[alexgenin]

Model states that are not included in any transition do not show up when plotting the model, e.g. :

mod <- camodel(
  transition(from = "rock", to = "juv",  
             ~ mu + dmu * q["kelp"] * ( 1 - q["kelp"] )), 
  transition(from = "juv", to = "kelp",  
             ~ G * ( 1 - c * q["kelp"]) ), 
  transition(from = "kelp", to = "rock", 
             ~ m + B * alpha * H * ( 1 - b * q["kelp"])),
  transition(from = "juv", to = "rock",  
             ~ m + B * ( 1 - alpha ) * H * ( 1 - b * q["kelp"] )), 
  neighbors = 8, 
  wrap = TRUE, 
  all_states = c("juv", "kelp", "rock", "sand", "missing"), 
  parms = list(m = 0.00055, mu = 0.2, dmu = 0.8, b = 0.67, alpha = 0.1, B = 0.045, G = 0.657, H = 0.1, c = 0.715)
)
plot(mod)

This snippet should display also 'sand' and 'missing' in the graph, but it does not.