Bugfix/accumulators queries on tsdb

Describe the MR

Fix the way data are shaped in the front app to be displayed in graph.

Why was this MR needed?

In the accumulator case, the first point was undefined, which lead to the graph not being rendered at all.

Extra-care points?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• CHANGELOG updated
• Unit Tests added/updated

What are the relevant issue numbers?

N/A

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