rPref (version 1.0.0)

get_btg: Better-Than-Graph

Description

Returns a Hasse diagram of a preference order (also called the Better-Than-Graph, short BTG) on a given data set. Either uses the igraph package or generates output for graphviz/DOT.

Usage

get_btg(df, pref, flip.edges = FALSE)
plot_btg(df, pref, labels = 1:nrow(df), flip.edges = FALSE)
get_btg_dot(df, pref, labels = 1:nrow(df), flip.edges = FALSE, file = NULL)

Arguments

df

A data frame.

pref

A preference on the columns of df, see psel for details.

flip.edges

(optional) Flips the orientation of edges, if TRUE than arrows point from worse nodes to better nodes.

labels

(optional) Labels for the vertices. Default values are the row indices.

file

(optional) If specified, then get_btg_dot writes the graph specification to given file path. If not specified, the graph specification is returned as a string.

DOT (Graphviz) Output

The function get_btg_dot produces the graph specification of the Better-Than-Graph in the DOT language of the Graphviz software. To produce the graph from that, you need the DOT interpreter. Depending on the file parameter the output is either written to a file or returned as a string. As the DOT layouter is suited for strict orders, the layouts of strict oder preference are generelly better than those generated with igraph. DOT ensures that all edges are oriented in the same direction and the number of overlaps is low.

Additional Parameters

By default, the arrows in the diagram point from better to worse nodes w.r.t. the preference. This means an arrow can be read as "is better than". If flip.edges = TRUE is set, then the arrows point from worse nodes to better nodes ("is worse than"). In any case, the better nodes are plotted at the top and the worse nodes at the bottom of the diagram. The names of the vertices are characters ranging from "1" to as.character(nrow(df)) and they correspond to the row numbers of df. By default, these are also the labels of the vertices. Alternatively, they can be defined manually using the labels parameter of plot_btg or get_btg_dot.

Details

The function get_btg returns a list l with the following list entries:

l$graph

An igraph object, created with the igraph package.

l$layout

A typical Hasse diagram layout for plotting the graph, also created with igraph.

To plot the resulting graph returned from get_btg, use the plot function as follows:

plot(l$graph, layout = l$layout).

For more details, see igraph.plotting and the examples below. The function plot_btg directly plots the Better-Than-Graph some defaults values for e.g., vertex size, using igraph.

The Hasse diagram of a preference visualizes all the better-than-relationships on a given data set. All edges which can be retrieved by transitivity of the order are omitted.

See Also

igraph.plotting

Examples

# pick a small data set and create preference and BTG 
df <- mtcars[1:10,]
pref <- high(mpg) * low(wt)

# directly plot the BTG with row numbers as labels
plot_btg(df, pref) 

# create the BTG and labels for the nodes with relevant values
btg <- get_btg(df, pref)
labels <- paste0(df$mpg, "\n", df$wt)

# plot the graph using igraph
library(igraph)
plot(btg$graph, layout = btg$layout, vertex.label = labels,
     vertex.size = 25)
     
# Create a graph with Graphviz (requires installed Graphviz)
## Not run: 
# # creates tmpgraph.dot in the current working directoy
# get_btg_dot(df, pref, labels, file = "tmpgraph.dot")
# # convert to diagram tmpgraph.png using Graphviz
# shell(paste0('"C:/Program Files (x86)/Graphviz2.38/bin/dot.exe"',
#              ' -Tpng tmpgraph.dot -o tmpgraph.png'))
# # open resulting image
# shell("tmpgraph.png")## End(Not run)


# show lattice structure of 3-dimensional Pareto preference in igraph
df <- merge(merge(data.frame(x = 1:3), data.frame(y = 1:3)), data.frame(z = 1:2))
labels <- paste0(df$x, ",", df$y, ",", df$z)
btg <- get_btg(df, low(x) * low(y) * low(z))
plot(btg$graph, layout = btg$layout, vertex.label = labels, 
     vertex.size = 20)