Charting data points and show a target range on the same chart.

Can anyone offer advise on how to make a chart of data points (line) with in
a particular range of target values? Speciffically, I am try to chart the pH
values of my fish tank. I know how to plot the data points on a line graph,
but I'd like to add a visual "highlighted" target range section. Ie, my data
records the pH for three days (8.2, 8.4, 8.5). pH on the y-axis and the date
on the x-axis. The acceptable range for the pH values for a sal****er tank
is 8.1 - 8.5 How can I indicate this target range within the chart? Once I
learn how to do this I can apply it to other tests (nitrates etc..)
i'd sure appreciate some help...