Adding a Baseline Value into my existing column graph.
 

I am constructing "custom-column-with 3D visual effect" graphs. For example I
have concentration on the y-axis and time on the x-axis. I now would like to
add a horizontal line, parallel to my x-axis, at a certain baseline
concentration. So that when you look at the graph you can compare the actual
concentration value on a given date to the baseline value, in my case this
base line would be some government acceptable concentration.