Emphasis Controls in a line graph would allow card creators to choose which line (from a Series of lines) should be emphasized. I think this is most valuable if the emphasis is "locked" (can't be changed by users). Alternatively, it could act as a "default" emphasis (allowing users to change emphasis by hovering over other lines or the legend).
Today, this emphasis can be added by user actions, but it isn't preserved by default or shown to other users. If a line graph has a Series of lines, users can hover over one line or an entry in the Legend. Doing so emphasizes that line. Other lines turn a light grey color, and their value labels are hidden. The emphasized line has value labels added.
This emphasized view is a classic strategy for data visualization. In the textbook "Stortytelling with Data" with Cole Nussbaumer Knaflic, Chapter 4 is dedicated to this kind of strategy. Academic research into the psychology of vision has proven that certain pre-attentive attributes (like color, hue, and size) in data visualizations will capture a user's attentive immediately and with great effect. This medium article also discusses some examples, including a line graph with emphasis: https://medium.com/microsoft-power-bi/data-storytelling-101-the-magic-of-pre-attentive-attributes-522da9785f36
This is especially powerful when we want to visualize "noisy" data. Imagine a line graph with 100 lines. It's visually confusing, overwhelming, with no clear focus. Viewers likely can't read data labels. A lot of data is visually "lost" due to overlaps. Emphasizing just one of these lines allows us to tell a clear story: this might represent an average, a new methodology, or an outlier. And the rest of the data is preserved for context, making it clear how the data we're emphasizing relates to the rest of the data we have.
Currently, Analyzer can get close to this functionality if we add color rules turning all lines grey and then a rule coloring our preferred line. What this can't achieve is allowing data labels on just the emphasized line, though. A control in analyzer to emphasize one line in a line graph would support this classic data viz strategy, empowering better storytelling.
