Fix bug causing order of parameters to get changed

I believe the goal here is to remove the unused levels of the factor. droplevels(D.sub$Parameter) should be used for this, because the use of factor(as.character(D.sub$Parameter)) was causing the existing ordering of the levels of Parameter to be lost and to be replaced with a purely alphabetical ordering. This was leading to "x[10]" coming before "x[2]" for example. As a result, in parameters were in the wrong order in many of the ggs_ plotting functions if the "family" option was used.
Merge pull request #37 from akrawitz/patch-1

Fix bug causing order of parameters to get changed (i.e. alpha[10] comes before alpha[1]).