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Feature Description: Add the ability to fill in line markers with color and also edit the fills opacity

• Additional context: Please add any additional information you feel will help us understand the feature

Not sure I understand the requirement - line markers are one or more line segments: there really isn't an interior to fill or be opaque. Area markers are basically the version of line markers that have an interior (by virtue of the last corner closing the cool formed by the other corners). Am I missing something?