Okay, so here's the situation:

We've got an Excel chart displaying calculated values on the y-axis and the
day of the analysis on the x-axis.

I'd like to add lines on this graph displaying the mean and then lines for +
and - the standard deviation to show how each dot's represented value
compares to the overall average.

It'll just be as simple as y = 75, where the number (in this case, 75) will
change every day as the mean is recalculated due to new points being entered.
Is there any way to get the graph to look at the cell that's displaying the
value of the mean already?