Can gnuplot compute and plot the delta between consecutive data points

dtop's solution didn't work for me, but this works and is purely gnuplot (not calling awk):

delta_v(x) = ( vD = x - old_v, old_v = x, vD)
old_v = NaN
set title "Compute Deltas"
set style data lines
plot 'data.dat' using 0:($1), '' using 0:(delta_v($1)) title 'Delta'

Sample data file named 'data.dat':

0
1
4
9
16
25

Here's how to do this without pre-processing:

Script for gnuplot:

# runtime_delta.dem script
# run with 
#     gnuplot> load 'runtime_delta.dem'
#
reset

delta_v(x) = ( vD = x - old_v, old_v = x, vD)
old_v = NaN

set title "Compute Deltas"
set style data lines

plot 'runtime_delta.dat' using 0:(column('Data')), '' using 0:(delta_v(column('Data'))) title 'Delta' 

Sample data file 'runtime_delta.dat':

Data
0
1
4
9
16
25

How about using awk?

plot "< awk '{print $1,$1-prev; prev=$1}' <datafilename>"