The "average" value for the transaction error rate chart seems to include null values.
Selecting this week shows a 3.7% average:

Zooming in on the area with data shows a 7.2% average:

The API response includes an average value which seems to be more correct, but it is ignored in the client and re-calculated there. See x-pack/plugins/apm/public/components/shared/charts/ErroneousTransactionsRateChart/index.tsx.
The "average" value for the transaction error rate chart seems to include null values.
Selecting this week shows a 3.7% average:

Zooming in on the area with data shows a 7.2% average:

The API response includes an
averagevalue which seems to be more correct, but it is ignored in the client and re-calculated there. Seex-pack/plugins/apm/public/components/shared/charts/ErroneousTransactionsRateChart/index.tsx.