How can I find the value difference between two data series on an Excel scatter graph, when both of the series have different timestamp entries for the X-axis? For example:

Series 1
11:20:02 333
11:20:15 332
11:20:28 325

Series 2
11:20:04 992
11:20:11 944
11:20:18 932

Therefore I need to find the difference between the lines which have been plotted rather than the actual data-points... Is this even possible in Excel?