ENH: pass color to fixation
RF: agg methods moved outside Plot class
NF: get_ax() for getting the current axis
ENH: subplotno is the current subplot until next plot called
ENH: error handling improved
cmd will only accept either the first letter of an arg, or the full name
BF: x-axis labels fixed
ENH: x-axis labels rotate only when longer than 20 chars
BF: dissimilarity now actually returns the dissimilarity measure