support precipitation ph and ph_method special plot case: 

I need to edit
the requirement for pH graphs. The verticle lines should differ if the
measurement was taken with pH paper. In tho case they should extend from
.5 below the reported vale to
.5 above. So for instance, a value of 7 measured with paper would be
graphed as a verticle line from 6.5 to 7.5 while the same value measured
with pH pen or meter would extend from 6.9 to 7.1