Hi,

try this:

add a new column with dummy data to your data table, numbers don't matter
add this new data to your chart
select the new data series and open the format dialog
on the Axis tab select "secondary axis"
now you see the second y axis on the graph
set all the values for your new dat series to zero, so they don't show
format the secondary axis to have the same scale and marker settings as your primary axis
delete the new data series from the legend.

does that help?