How will you highlight the points? If you want to place a line (like a
gridline) at a given date, try a combination of these two techniques:
http://peltiertech.com/Excel/Charts/AddLine.html
http://peltiertech.com/Excel/Charts/ArbitraryGridlines.html
If you want a label on the appropriate points, use formulas to put the
labels into cells in a range next to the values, then use Rob Bovey's
Chart Labeler addin or John Walkenbach's Chart Tools addin to apply
labels from that range to the chart series.
If you want a custom marker, use the data in the other sheet to plot
another series, with different markers that reside over the point they
are highlighting in the original series.
- Jon