visualisation of joint and marginal distributions in STV preferences
# S3 method for SafeVote.stv
image(x, xpref = 2, ypref = 1, all.pref = FALSE, proportion = TRUE, ...)
image object, with side-effect in RStudio Plots pane
STV results to be visualised
candidates shown in a joint distribution plot
plot the joint distribution of two preferences (if 'all.pref=FALSE') or the marginal distribution of all preferences (if 'all.pref=TRUE').
The joint distribution can be shown either as proportions (if 'proportion=TRUE') or raw vote counts (if 'proportion=FALSE').
args passed to fields::image.plot()