mummerplot:ordering reference sequences on the X-axis

[Mahul Chakraborty]

Hi,
I am using mummer (nucmer) to create a dotplot between the genome assemblies of two closely related species. Then I am using mummerplot to view the plot. However, I am unable to arrange the reference contigs in a particular order on the X-axis. Even when the reference contigs are present in the order R1,R2,R3,R4 in the sequence fasta file, mummerplot arranges them as R1,R2,R4,R3 on the X axis of the mummerplot. Is there a way to work around this? I tried using the -R switch but that didn't solve the issue.
mummerplot command I am using:

mummerplot -fat -filter -large out.delta

Thank you.
Mahul