How can I find out the code DRIMSeq uses to make dmplotProportions objects?
I was curious how they have done it as well, so here expanded @Liopis comment :

wget http://bioconductor.org/packages/release/bioc/src/contrib/DRIMSeq_1.6.0.tar.gz
tar -zxf DRIMSeq_1.6.0.tar.gz 
# if you need to find it, or use grep
# find DRIMSeq -name "dm_plotProportions.R"
less DRIMSeq/R/dm_plotProportions.R