There are many possible interpolation algorithms. Perhaps the simplest is a linear one.
Suppose your dates (in ascending order) are in column A and the data are in column B and there are gaps in column B between rows 9 and 19. To generate a linear interpolation for the missing data, you could insert the following formula in B10 and copy down to B18:
=(B$9-B$18)/(A$9-A$18)+B9

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Thanks for your thoughtful response, but it didn't work for me. Maybe I did something wrong, but tried several times.

Actually, I probably phrased my question incorrectly. I'm looking for a simple solution and don't need interpolation. I was thinking more along the lines of "if a cell is zero or blank, insert previous cell contents." That would keep the line graph from bottoming out and giving the appearance that investments went to zero on the days where there is no data entered.

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