[BioC] Pairs plots in lumi, plots look different?

[Julien Bauer]

Hello,
I am working for a microarray facility at Cambridge University, we have 
been using the Illumina platform for a while now.
Lumi is a great package but I noticed something rather odd, when using 
the plot function "pairs" on my data, if I run it again the plots look 
different, some points are shifted or change location. The rest stay the 
same it just the look of the graphs that change.
My guess is that it is because of the auto scaling of the graph but I 
would like to be sure. I look in the mailing list archive and in the 
vignette but I couldn't find the answer for this.
Thanks in advance for your help,

Julien Bauer