Expand comparison plotting to 2+ endpoint samples per plot

[madison-feshuk]

Currently comparison plotting is limited to dual plots (i.e. where ordered m4ids supplied and the length of compare.val and val must be equal)

A few users have developed their own standalone plotting to plot 2+ endpoint samples per plot, but it would be nice to introduce this capability to tcpl. Some examples:

• ATG mixtures (3)
  
• Deisenroth DEVTOX (4)

Consider adding "compare.fld" to group by spid or endpoint. Will not require dat.
Consider overlay of values and cutoff

• Rules based on number of curves per plot

[cthunes]

Two options for choosing a compare field:

compare (or compare.fld) parameter:

• string specifiying column name - m4id default (aeid, dtxsid/chnm, spid, custom/generic char column is fine)
• groups into list of data frames
  or user can
• supplied list of data of frames means it is already grouped and ready for ggplot function
• errors if dat is a list and compare.fld is set

prepared data: (not doing this now)

• dplyr grouping:
  based on if dat is supplied

[cthunes]

Make it extensible past 26 by creating a function to end LETTERS like A,B,C........AA,AB,AC.......BA,BB,BC.........ZX,ZY,ZZ,AAA,AAB
If number of curves exceeds 8:

• Consider dropping all title (except for those in common across all) and legend
• All points could be black, curves could be grouped in color by a specified variable (or parameter) with default like 'modl' (all poly2 the same color, poly1 the same, etc)
• This number of curves could also be a parameter with default 8 but settable

[cthunes]

If number of curves exceeds 2 or maybe 3:

• Move title components into verbose table