Here's a walkthrough of the problem:

I add a new row in Zoho Docs sheet. Note columns R, S & T show data as %. 

Columns R, S & T are calculated columns. Here is the formula for column R. It's essentially calculating the ratio between the sum of the previous 52 rows in column G and the current row Column P, which, given the nature of the data, always calculates to a decimal <=1 and >0. In the case of the screenshot above, it's 0.4048, or 40.48%.

When Zoho Reports automatically fetches the sheet into the appropriate Reports table on its regular schedule, this is what it looks like. Notice the last row in the "Guest Volume" column. This is the same data represented above in the screenshot of the Spreadsheet. It should be 40.48%, but it's coming in to Reports as 0.4%

Thus, the graphs on my dashboard look like this.

I can correct the error by going back to the sheet and reapplying the % formatting to columns R, S & T (select the columns, then choose from the top menu Format>%) ...

...then, I go to Reports and refetch the spreadsheet (which is set to overwrite all data on refetch). Now the data comes through properly (40.5%). It doesn't matter how many times I refetch the data, it comes through correctly UNTIL I add a new row to the spreadsheet in Zoho Docs Sheets.

And now my charts appear correctly: