Graphing data with compliance limit

Currently I graph data from laboratory tests with results in the y-axis and
dates in the x-axis. I then create another series to display the compliance
level for the test over the same time period. To achieve this I need to
create another column on my data sheet to include the compliance result in
each row.

Is it possible to have the compliance figure in a single cell on my data
sheet, yet still display the compliance level on my graph together with my
data points?

yes, calculate the compliance level putting the figure in, say, cell A1

Do your chart as normal

Highlight your chart and in the formula bar type =

then go to cell A1 and press return, and locate, format as desired

You now have a permanent link even when you amend your data