An issue with argument "sortv" of function seqIplot() - traminer

Question

I'm trying to plot individual sequences by means of function seqIplot() in TraMineR. These individual sequences represent work trajectories, completed by former school's graduates via a WEB questionnaire.
Using argument "sortv", I'd like to sort my sequences according to the order of the levels of one covariate, the year of graduation, named "PROMO".
"PROMO" is a factor variable contained in a data frame named "covariates.seq", gathering covariates together:
str(covariates.seq)
'data.frame': 733 obs. of 6 variables:
$ ID_SQ : Factor w/ 733 levels "1","2","3","5",..: 1 2 3 4 5 6
7 8 9 10 ...
$ SEXE : Factor w/ 2 levels "Féminin","Masculin": 1 1 1 1 2 1
1 2 2 1 ...
$ PROMO : Factor w/ 6 levels "1997","1998",..: 1 2 2 4 4 3 2 2
2 2 ...
$ DEPARTEMENT : Factor w/ 10 levels "BC","GCU","GE",..: 1 4 7 8 7 9
9 7 7 4 ...
$ NIVEAU_ADMISSION: Factor w/ 2 levels "En Premier Cycle",..: NA 1 1 1 1
1 NA 1 1 1 ...
$ FILIERE_SECTION : Factor w/ 4 levels "Cursus Classique",..: NA 4 2 NA
1 1 NA NA 4 3 ..
I'm also using "SEXE", the graduates' gender, as a grouping variable. To plot the individual sequences so, my command is as follows:
seqIplot(sequences, group = covariates.seq$SEXE,
sortv = covariates.seq$PROMO,
cex.axis = 0.7, cex.legend = 0.7)
I expected that, by using a process time axis (with the year of graduation as sequence-dependent origin), sorting the sequences according to the order of the levels of "PROMO" would give a plot with groups of sequences from the longest (for the older graduates) to the shortest (for the younger graduates).
But I've got an issue: in the output plot, the sequences don't appear to be correctly sorted according to the levels of "PROMO". Indeed, by using "sortv = covariates.seq$PROMO" as in the command above, the plot doesn't show groups of sequences from the longest to the shortest, as expected. It looks like the plot obtained without using the argument "sortv" (see Figures below).
Without using argument "sortv"
Using "sortv = covariates.seq$PROMO"
Note that I have 733 individual sequences in my object "sequences", created as follows:
labs <- c("En poste","Au chômage (d'au moins 6 mois)", "Autre situation
(d'au moins 6 mois)","En poursuite d'études (thèse ou hors
thèse)", "En reprise d'études / formation (d'au moins 6 mois)")
codes <- c("En poste", "Au chômage", "Autre situation", "En poursuite
d'études", "En reprise d'études / formation")
sequences <- seqdef(situations, alphabet = labs, states = codes, left =
NA, right = "DEL", missing = NA,
cnames = as.character(seq(0,7400/365,1/365)),
xtstep = 365)
The values of the covariates are sorted in the same order as the individual sequences. The covariate "PROMO" doesn't contain any missing value.
Something's going wrong, but what?
Thank you in advance for your help,
Best,
Arnaud.

Answer 1

Using a factor as sortv argument in seqIplot works fine as illustrated by the example below:
sdc <- c("aabbccdd","bbbccc","aaaddd","abcabcab")
sd <- seqdecomp(sdc, sep="")
seq <- seqdef(sd)
fac <- factor(c("2000","2001","2001","2000"))
par(mfrow=c(1,3))
seqIplot(seq, with.legend=FALSE)
seqIplot(seq, sortv=fac, with.legend=FALSE)
seqlegend(seq)