plot_policy_graph: POMDP Plot Policy Graphs

Description

The function plots the POMDP policy graph for converged POMDP solution and the policy tree for a finite-horizon solution.

Usage

plot_policy_graph(
  x,
  belief = NULL,
  engine = c("igraph", "visNetwork"),
  show_belief = TRUE,
  state_col = NULL,
  legend = TRUE,
  simplify_observations = TRUE,
  remove_unreachable_nodes = TRUE,
  ...
)
curve_multiple_directed(graph, start = 0.3)

Value

returns invisibly what the plotting engine returns.

Arguments

x: object of class POMDP containing a solved and converged POMDP problem.
belief: the initial belief is used to mark the initial belief state in the graph of a converged solution and to identify the root node in a policy graph for a finite-horizon solution. If NULL then the belief is taken from the model definition.
engine: The plotting engine to be used.
show_belief: logical; show estimated belief proportions as a pie chart or color in each node?
state_col: colors used to represent the belief over states in each node. Only used if show_belief is TRUE.
legend: logical; display a legend for colors used belief proportions?
simplify_observations: combine parallel observation arcs into a single arc.
remove_unreachable_nodes: logical; remove nodes that are not reachable from the start state? Currently only implemented for policy trees for unconverged finite-time horizon POMDPs.
...: parameters are passed on to policy_graph(), estimate_belief_for_nodes() and the functions they use. Also, plotting options are passed on to the plotting engine igraph::plot.igraph() or visNetwork::visIgraph().
graph: The input graph.
start: The curvature at the two extreme edges.

Details

The policy graph returned by policy_graph() can be directly plotted. plot_policy_graph() uses policy_graph() to get the policy graph and produces an improved visualization (a legend, tree layout for finite-horizon solutions, better edge curving, etc.). It also offers an interactive visualization using visNetwork::visIgraph().

Each policy graph node is represented by an alpha vector specifying a hyper plane segment. The convex hull of the set of hyperplanes represents the the value function. The policy specifies for each node an optimal action which is printed together with the node ID inside the node. The arcs are labeled with observations. Infinite-horizon converged solutions from a single policy graph. For finite-horizon solution a policy tree is produced. The levels of the tree and the first number in the node label represent the epochs.

For better visualization, we provide a few features:

Show Belief, belief color and legend: A pie chart (or the color) in each node can be used represent an example of the belief that the agent has if it is in this node. This can help with interpreting the policy graph. The belief is obtained by calling estimate_belief_for_nodes().
Simplify observations: In some cases, two observations can lead to the same node resulting in two parallel edges. These edges can be collapsed into one labels with the observations.
Remove unreachable nodes: Many algorithms produce unused policy graph nodes which can be filtered to produce a smaller tree structure of actually used nodes. Non-converged policies depend on the initial belief and if an initial belief is specified, then different nodes will be filtered and the tree will look different.

These improvements can be disabled using parameters.

Auxiliary function

curve_multiple_directed() is a helper function for plotting igraph graphs similar to igraph::curve_multiple() but it also adds curvature to parallel edges that point in opposite directions.

Examples

Run this code

data("Tiger")

### Policy graphs for converged solutions
sol <- solve_POMDP(model = Tiger)
sol

policy_graph(sol)

## visualization
plot_policy_graph(sol)

## use a different graph layout (circle and manual; needs igraph)
library("igraph")
plot_policy_graph(sol, layout = layout.circle)
plot_policy_graph(sol, layout = rbind(c(1,1), c(1,-1), c(0,0), c(-1,-1), c(-1,1)), margin = .2)
plot_policy_graph(sol,
  layout = rbind(c(1,0), c(.5,0), c(0,0), c(-.5,0), c(-1,0)), rescale = FALSE,
  vertex.size = 15, edge.curved = 2,
  main = "Tiger Problem")

## hide labels, beliefs and legend
plot_policy_graph(sol, show_belief = FALSE, edge.label = NA, vertex.label = NA, legend = FALSE)

## custom larger vertex labels (A, B, ...)
plot_policy_graph(sol,
  vertex.label = LETTERS[1:nrow(policy(sol))],
  vertex.size = 60,
  vertex.label.cex = 2,
  edge.label.cex = .7,
  vertex.label.color = "white")

## plotting the igraph object directly
pg <- policy_graph(sol, show_belief = TRUE, 
  simplify_observations = TRUE, remove_unreachable_nodes = TRUE)

## (e.g., using a tree layout)
plot(pg, layout = layout_as_tree(pg, root = 3, mode = "out"))

## change labels (abbreviate observations and use only actions to label the vertices)
plot(pg,
  edge.label = abbreviate(E(pg)$label),
  vertex.label = V(pg)$action,
  vertex.size = 20)

## use action to color vertices (requires a graph without a belief pie chart) 
##    and color edges to represent observations.
pg <- policy_graph(sol, show_belief = FALSE, 
  simplify_observations = TRUE, remove_unreachable_nodes = TRUE)

plot(pg,
  vertex.label = NA,
  vertex.color = factor(V(pg)$action),
  vertex.size = 20,
  edge.color = factor(E(pg)$observation),
  edge.curved = .1
  )

acts <- levels(factor(V(pg)$action))
legend("topright", legend = acts, title = "action",
  col = igraph::categorical_pal(length(acts)), pch = 15)
obs <- levels(factor(E(pg)$observation))
legend("bottomright", legend = obs, title = "observation",
  col = igraph::categorical_pal(length(obs)), lty = 1) 

## plot interactive graphs using the visNetwork library.
## Note: the pie chart representation is not available, but colors are used instead.
plot_policy_graph(sol, engine = "visNetwork")

## add smooth edges and a layout (note, engine can be abbreviated)
plot_policy_graph(sol, engine = "visNetwork", layout = "layout_in_circle", smooth = TRUE)


### Policy trees for finite-horizon solutions
sol <- solve_POMDP(model = Tiger, horizon = 4, method = "incprune")

policy_graph(sol)

plot_policy_graph(sol)
# Note: the first number in the node id is the epoch.

# plot the policy tree for an initial belief of 90% that the tiger is to the left
plot_policy_graph(sol, belief = c(0.9, 0.1))

# Plotting a larger graph (see ? igraph.plotting for plotting options)
sol <- solve_POMDP(model = Tiger, horizon = 10, method = "incprune")

plot_policy_graph(sol, edge.arrow.size = .1,
  vertex.label.cex = .5, edge.label.cex = .5)

plot_policy_graph(sol, engine = "visNetwork")

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