xyplot (lattice), strip.default

Dear r-help,

  I've got data of the following structure
1979  93.428747  0
1979  87.298608  20
1979  78.506340  40
...
1979  45.567890  340
1980  60.815289  0
1980  49.630904  20
1980  24.981362  40
...

The first column is year and the last one is the longitude.
I need a set of graphs showing the dependence of the middle value on
the longitude, for each year, situated one blow the other.
I.e.
1979: --...---```--``--..
1980: ...--``../``\...---
...
etc.

Here characters ---...---``` denote the curve.
The middle value is on the vertical axis
and the latitude is on the horizontal axis of each graph.

To do this I use xyplot function from the Lattice package:

xyplot(ac15$"value"~ac15$lon|year,data=ac15,
       type="l",
       layout=c(1,24),
       xlab="Longitude",
       as.table=TRUE,
       bg="white"
      );

But I have some problems using it.

Questions:

1. How to make xyplot to draw the year value in the strip above each
graph instead of writing the word 'year'?