Plot an explicit point next to a graph

Hello,

I'd like to plot a graph from a data file with lines and then plot one
point that is not in the data with points on the same graph.

If I try something like:
plot "t.dat" using 1:2 title "Data" with lines, \
    10 with points

Then I get a horizontal line in 10 with points.

If I try to limit the xrange for the second plot I get an "invalid
expression error"
plot "t.dat" using 1:2 title "Data" with lines, \
    [4:5] 10 with points

Any ideas on how to do that?

Thanks,
Miki