Custom Line Style (Dashed/Solid/Dotted) per Series in ECharts Line & Mixed Charts

[simon-cerda]

Is your feature request related to a problem? Please describe.

Currently, in Apache Superset (tested on 4.x, 5.x, and checking 6.0.x beta), the "Dashed" line style is exclusively reserved for Time Shift comparisons (e.g., "vs 1 year ago").

There is no native way in the UI to manually set a specific metric (series) to be displayed as a dashed line.
This is a critical limitation for business dashboards that need to compare Actual vs. Budget/Target or Forecast vs. Real, where the standard convention is to use a solid line for "Actual" and a dashed line for "Budget/Target" without relying on time-shifts.

Describe the solution you'd like

I propose adding a control in the Customize tab (or within the specific Series configuration in Mixed Charts) to override the lineStyle.

Preferred UX:
In the "Customize" tab, under a new "Series Style" section:

1. Select the Metric/Series name.
2. Select Line Type: Solid (Default) | Dashed | Dotted.

Technical Context:
Since Superset uses Apache ECharts, this is natively supported by the library via series.lineStyle.type.
Currently, transformProps.ts seems to hardcode this logic based solely on the isTimeShift boolean. We need a way to pass a manual override prop to the ECharts generator.

Describe alternatives you've considered

1. Time Shift Hack: Users are currently forcing fake time comparisons to get dashed lines, which messes up the tooltips and legend labels.
2. CSS Hacks: Impossible because ECharts renders on Canvas, not DOM elements.
3. JSON Metadata: There is no exposed property in the Chart's JSON metadata to inject custom series styles in the current versions.

Additional context

As Superset moves towards v6.0 with the Ant Design system, adding granular control over visual data representation would be a huge win for data storytelling.

We are looking to implement this for charts like:

• Revenue (Solid) vs. Revenue Goal (Dashed)
• Temperature (Solid) vs. Threshold (Dashed)