Delete selected major gridlines in a chart

[Caldo]

I have an x axis with the values -50, -40, -30, -20, -10, 0, 10, 20, 30 with
all values in %. I would like for only gridlines to be displayed at the -20
and 0 points along the axis. How can I do this? I have not found a solution
yet searching on the web. thanks

 

[Jon Peltier]

Gridlines come as a set. You can remove them all, assume the crossing
axis can represent the 0 value, and use one of these techniques to add a
line at -20:

Add a Line to a Column or Line Chart
http://peltiertech.com/Excel/Charts/AddLine.html

- Jon