changing order of txt stamp and main plot

[tokami]

allows plotting in same coordinate system as main plot.

Note: It is not possible to outsource

        if (!is.null(stamp) && stamp != '' && !is.na(stamp)){
            txt.stamp(stamp, new=FALSE)
            opar <- par(new = TRUE)
            on.exit(par(opar))
        }

to a separate function as the par needs to be defined on the same level as the par of the main plot. Thus there is a bit of code repetition...