The question marks in the plot are almost certainly an issue with Gnuplot not having the right font set.  I think we'd need to know what OS you were using to give further guidance, though I know very little about the internal working of Gnuplot myself.

The file you attached is not an XLS file, but a plain text file.  As Fred pointed out, it's got an absurd number of zeros, with each value having more than 60 decimal places.  On the first line, the values represented span 23 order of magnitude.  That's all pretty impressive, really, but you might want to encourage the folks in charge of the data collection system to reconsider their approach.  And, they should include labels and some documentation for the file!

It also looks like the data is actually fluorescence data. I'm guessing column 2, 3, and 4 are all somehow measuring I0 (though 4 is noisier, it varies the same way), all possibly in Amps, and that column 5 is fluorescence measured as an ROI from a solid-state detector, with column 5 being some measure of total counts in that detector, or possibly some counts from a fluorescence ion chamber -- the scale doesn't really make sense to me, but I could not guess the time per point.

Anyway, the data is definitely in fluorescence, but what you show makes it look like the log was taken when reading in this data -- an easy mistake to make when reading in data with Athena.

--Matt