axes on time-series plots

The following code uses the internal numerical representation of the dates (of class Date) rather than annotating the axes using e.g. the year in this case

```
library(imp.lattice)
x11()
.impenv$backend <- graphics_primitives
data(economics, package = "ggplot2")
p <- xyplot(uempmed ~ date, data = economics,
            pch = 19, xlab = "date", ylab = "uempmed") + ypanel.lines()
p
```
