Can we use different line thicknesses in a line chart to make a line prominent and easier to catch? The goal is to make one of the lines stand out from the rest (say average traffic).
This chart may have data for 4 lines... I know it becomes less readable but that's the use case...so the lines can be turned on and off using legends to make it less cluttered.
It's common in data visualizations to emphasize one metric in particular, which is referred to as a reference line.
From Data Visualization, by Andy Kirk:
Line charts or scatter plots are particularly enhanced by the inclusion of reference lines, helping to direct the eye towards calculated trends, constants or averages...
Line charts like the one you posted get a nickname once they reach even more data points: the spaghetti chart. Nothing stands out.
Emphasizing the most important metric is perfectly acceptable, provided you know your users needs (the most salient metric they should be paying attention to).
You can make the line thicker, as well as use a more muted color palette for other data points as well:
Yes, and basically I suggest you to highlight the option user both hovered on and clicked besides providing users to choose among others as an option. Let me demonstrate it with a visual and a link from my own portfolio even it's a bit older.
Via the illustration above, when the user clicks the small squares under the chart labeled with "A, B, C, and D" it's possible to include or exclude the same colored stacks on the chart. Even it's a basic instinct level practice, I'll share the link here for the users willing to see the interaction incase I can't explain the situation satisfactorily.
