Make target line not mandatory for hourly_stacked_area charts

[aaccensi]

Bug Description

The hourly_stacked_area charts seem to require one of the series to be configured as "target line" for the chart to work.

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Screenshots

In the screenshot above the residual load (red line) is the target line.

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Steps to Reproduce

1. In ETModel, navigate to Flexibility > Electricity Storage.
2. Observe the chart: Flexible electricity supply and demand per hour

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Expected interaction

Ideally it shouldn't be always necessary to have a target line cluttering the UI if it has no purpose.

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Proposed solutions

Investigate why this is required, and if we can change that behaviour.

[github-actions]

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