RRD Graph downward to 0 date prediction

Good morning,

I'm working through doing a low-disk space prediction (based on slope)
I see a lot of examples floating around of doing a 90% and 100% 
prediction, however nothing for 0 prediction.

I'm extremely close to the final prediction and have a nice slope 
line... however I just can't seem to get the
prediction date to work:

#{c} is a string such as "storage01"
#.. is a commented out line (this is all ruby)

The final "GPRINT:#{c}warning:    2TB free @ %c\\l:strftime" is the 
final prediction
that isn't functioning. (I just get a "-")

     # A nice 500GB / 2TB limit red
     options.push("LINE1:0")
     options.push("AREA:500000000000#FF000044::STACK")
     options.push("AREA:2000000000000#FF000022::STACK")
     index = 0
     counters.each do |c|
          options.push("--end", "now+#{range}", "--start", 
"now-#{range}")
          options.push("--vertical-label", "Space Available")
          options.push("DEF:#{c}=#{rrdfile}:#{c}:MIN")
          options.push("VDEF:#{c}slope=#{c},LSLSLOPE")
          options.push("VDEF:#{c}int=#{c},LSLINT")
          
options.push("CDEF:#{c}avg=#{c},POP,#{c}slope,COUNT,*,#{c}int,+")
          options.push("CDEF:#{c}limit=#{c}avg,2000000000000,0,LIMIT")
          options.push("VDEF:#{c}warning=#{c}limit,FIRST")
          options.push("LINE2:#{c}#{get_color(index)}cc:#{c}")
          options.push("LINE1:#{c}avg#{get_color(index)}cc::dashes=2");
          options.push("GPRINT:#{c}:LAST:Available\\: %7.2lf %s\\l")
          #options.push("GPRINT:#{metric}:MIN:Min Available\\: %7.2lf 
%s")
          #options.push("GPRINT:#{metric}:MAX:Max Available\\: %7.2lf 
%s\\l")
          options.push("GPRINT:#{c}warning:    2TB free @ 
%c\\l:strftime")
          index += 1
     end
     options.push("--title", "#{(object[:name] or object[:hostname])} - 
Disksafe Storage Trending / Last #{range}")
     #options.push("--lower-limit", "0", "--upper-limit", 
"25000000000000")


Thoughts? Trying to predict when 2TB is going to be all that is 
remaining. All data is in bytes.


  -- Alex