How can I chart a multi-year time series in Excel?

I have a multi-year time series that I am charting. I can produce a simple
line graph with a time-scale X-axis no problem, but I would like a multi-line
chart, with each year being a different line. This would be a powerful way of
comparing year-on-year changes. The base data is a series of measurements
taken at irregular dates. For analysis purposes I am using the average value
halfway between each date pair.