chain.graph() produces a graph incompatible (current) petgraph

[dream-dasher]

Generating a graph of a chain with chain.graph() on string data produces the following structure:
petgraph::graph_impl::Graph<Vec<Option<std::string::String>>, f64

This does not seem to play nicely with petgraph bounds.

Looking at this example from your repo the code appears to require a petgraph feature which is no longer present (as of petgraph 0.6.3).

Notably, {:?} printing of the graph structure does produce valid seeming data and the markov production process with the chain appears to work appropriately. So the underlying data appears to be mostly correct.

(Great tool btw!)