Tableau is a bit tricky when it comes to handling Null values. See Handling Null Values | Tableau Software
You may want to use this formula in calculated field
If this doesn't work, you may want to look up this discussion.
Or, you can always replace blanks by 0 in your excel file before importing into Tableau.
Thank you for the answer Manas. Unfortunately two problems: 1) I don't want to average in zeroes, and 2) if I simply add them up, ignore the nulls, and divide by 3 I will get an inaccurate value -- for example, 70+70+(null) / 3 provides 46.7 as the answer, though the average should be 70 for this line.
It is not clear how your data is structured in your datasource but my suggestion is to have it normalized instead of having a crosstab like the table you post shows.
Something like this:
Time reading temperature 1 T1 71 1 T2 72 1 T3 70 2 T1 71 2 T2 71 2 T3 71 3 T1 71 3 T2 73 3 T3 72 4 T1 72 4 T2 72 4 T3 72 5 T1 72 5 T2 72 5 T3 71 6 T1 70 6 T3 75
Take a look at the attached workbook with the solution.
I hope this helps