It seems that my graph is ggparcoordnot suitable for drawing using discrete scale. When I do this:
ggparcoord(data = iris, columns = 1:4, groupColumn = "Species")
the output graph still colors the lines using a continuous scale (using factor levels Species).

I also tried a modified version of the trick scale_color_manualindicated here: color control in ggparcoord , to no avail.
ggparcoord(data = iris, columns = 1:4, groupColumn = "Species") +
scale_color_manual(values = c("setosa" = "red",
"versicolor" = "green",
"virginica" = "blue"))
But I get this error message: Error: Continuous value supplied to discrete scale.
I also tried .. + scale_color_discrete(): same error message.
I'm stumped ... even the examples on the ggparcorod cran page do not work:
data(diamonds, package="ggplot2")
diamonds.samp <- diamonds[sample(1:dim(diamonds)[1],100),]
ggparcoord(data = diamonds.samp,columns = c(1,5:10),groupColumn = 2)
Error message: Error: (list) object cannot be coerced to type 'double'
Edit
mapping=aes(color=as.factor(Species)), Didzis Elferts. scale_color_discrete
ggparcoord(data = iris, columns = 1:4, mapping=aes(color=as.factor(Species))) +
scale_color_discrete("Species", labels=levels(iris$Species) )
, scale_color_discrete("Species", labels=levels(iris$Species) ) ?

, ggplot2,
ggplot(data=iris, aes(x=Sepal.Width,y=Sepal.Length,color=Species)) + geom_point()
. ggparcoord?
R 3.2.1, ggplot2_2.0.0 GGally_1.0.0
sessionInfo()
!