In regards to graphs.  You do not want to show the -9 for one only wants
to graph the amounts that are measurable or trace.

Because of the 2 scenarios is why we have a _flag column in the dv so
that the vis application can determine what should be plotted or not. 
However, based off prior discussions it was determined that the vis
application could not decipher _flag values and amount values at the
same time because it does not know how to cross check against 2 data
table columns.  To account for this I assigned a value of 1/2 of the
minimum measurable amount to account for trace (.5 for snow depth and
.05 for liquid amounts) and -9 to account for "missing".

My suggestion is to set a limit in the application that creates the
graph to say that the y-axis minimum is allows 0 and so anything below 0
will get truncated.