You will have to make custom calculations per X point and provide the final value in the Graph item's DataSource. then you can add LineSeries displaying the value.
The calculation can be done in the data-retrieval method, as a calculated field per row of data, or as an expression of the series' X|Y property.
Such calculations are not included in the graph item's functionality. Please feel free to log a feature request
for having an option to add series based on specific formula and the graph's data.
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