To Add a Simple Line Through My Chart

All i am trying to do is add a line through my Excel Chart

so say...

500
400-----------------------------------
300 | |
200 | | |
100 | | |
----day 1, day 2, day 3, day 4, day5

i just need to add a line showing where 400 is. and it's been impossible to google it. >.< please help