Constant net_ensembles::dot_constants::EXAMPLE_DOT_OPTIONS

pub const EXAMPLE_DOT_OPTIONS: &str = "bgcolor=\"transparent\";\n\tfontsize=50;\n\t\
        node [shape=ellipse, penwidth=1, \
        fontname=\"Courier\", pin=true ];\n\tsplines=true;";

Example options. You are free to use your own. Search for graphviz.