I am using the same timechart search query:
'search
| timechart span=1d sum(xxx)"
when I set the time range picker to yesterday preset I get a different value to when I change the time to 'week to date' and view the stats table the value for yesterday - could there be a reason for this?
How is the value of xxx derived? Do you see the same issue using another numeric field with a known derivation, e.g. sum(_time)?
You may be receiving different events each time the search executes, but it's not possible to verify that without an understanding of your deployment.