Trying to Plot grid lines every 7 days

[Tim Pierce]

# see my original graph (a sin-wave over 30 days)

plot( 1:30, sin((1:30) .* (2 * pi / 30) ) )

grid on

set (gca, 'xtick', 1:7:30 )

# So far  puts grid line every 7 days - good!

# Now this screws it and the xticks jump back to every 5 days 

datetick ( 'ddd dd')

# So I re-add this - now the whole axis jumps to the right so the days are incorrect (not it only goes up to 20th day)

set (gca, 'xtick', 1:7:30 )

I want every day to be monday, or sunday, or whatever