I have a bunch of line charts that plot values versus dates, where the
dates can be arbitrary dates and are not necessarily contiguous (dates
could be missing). Here's a simple code script illustrating how the
charts are set up:

With Chrt.SeriesCollection.NewSeries
.XValues = "{""07/01/2007"",""07/02/2007"",...}"
.Values = "{10,20,...}"
End With

With this approach, dates are merely a sequence of labels (I actually
use excel names rather than coding x and y values in a text string as
shown here).

Now what I would like to do is be able to draw a vertical timeline(s)
associated with key dates. So if 7/4/2007 were a key date, I would
like to indicate that on the chart. My initial thought was to just
create a new series with 2 data points, the y-axis min and max values
for the key date and the series would draw a vertical line.
Unfortunately, I can't duplicate dates in these charts so a new data
series won't work.

No doubt others have had the need to draw timelines so I'm looking for
any suggestions on how I might do that in my application.

Denis