Anna, isn't this line just your category axis? Then just select the
gridlines and change their colour, e.g. light grey / dotted, to discriminate
from the cat axis
Regards,
Henk

" wrote:

This is probably a very easy question I just haven't spent a lot of
time with Excel charts. How do you change the style of 1 gridline on a
chart?

The data on the Y axis is from -2 to 6 and we would like the line at 0
to be darker than the other gridlines.

Thanks,
Anna