Chart: adding a median line

Wondering if anyone can help me add a median line to my attched chart? With an explanation if possible. Thank you.

You will not be able to add a datum line to that chart as it is a 3d chart and those type do not allow you to create chart combinations.

See here for information on adding dautm lines
http://www.andypope.info/charts/averageline.htm

You may wish to consider your choice of chart because as pretty as it may look it is really difficult to read.