[Gnuplot-info] getting the time value for time series data

[Andy S. <ae...@sn...>]

I have a question, which if the answer is "you can't do that", might
be a suggestion:

Let say I have a data file with some time data:

2004 11  1  100.0
2005  1 15   90.2
2005  3 28   60.4
2006  4 15   23.8, etc

I do:

set xdata time
set timefmt "%Y %m %d"
p "data" us 1:4

Now I define a function:

f(t) = a * exp( -(t - t0) / tc)

I can fit with this function, and over plot the fit with:

p "data" us 1:4, f(x)

In the above, the function is called with the time in seconds.

But now I wish I could plot the residuals.

The thing I tried first:

p "data" us 1:4, "" us 1:($4 - f($1))

This produces a plot, but f() is called with just the year - not
the time is seconds.

I wish there was some way to get the x time value.  Is there
already a way?  - I was unable to puzzle one out...

Two possible syntaxes that I've thought of:

allow "$x" in a using expression to evaluate to the time
(and $y if "set ydata time" is in effect")

p "data" us 1:4,"" us 1:($4 - f($x))

of define a function, xtim (and ytim), which could be called with the
data file line number if it needs to have an argument

p "data" using 1:4,"" us 1:($4 - f(xtim($0)))

Comments?
Thanks,
  Andy S.