XY scatter and Bar chart - secondary axis misaligned
 

hi all,
i am making a gant chart, with stacked bar chart (using hidden series).
x axis (bottom) is date, y axis (left) is task name (ie category axis).
i have overlayed an xy scatter (i had an extra column giving the
vertical coordinates for the scatter, 0.5 1.5 etc, y coordinates are
dates) , which has worked fine when i had 4 categories.
however, when i include all needed categories (28), the secondary y
axis (right) disappears, and the scaling changes so that the vertical
alignment of the xy points is misaligned.
i have tried everything to show the secondary y axis so i can change
scale, but no luck - any ideas please.
thanks.