I have a map that displays flight routes. Now I'd like to indicate that some routes are operated more frequently.
Example (flights per week):
How to visualize the frequency so that the user is able to distinguish between frequent and less frequent routes?
I can imagine various solutions, including
Are there best practices?
Summary
Stroke width and color gradient are good choices, because you are dealing with ordinal data. Dotted/dashed line would not be a good choice for ordinal data.
Detailed Answer
Since you want the user to be distinguish frequent from less frequent routes, you need to choose a visual property that represents ordinal data.
Ordinal data is typically separated into natural, ordered categories, where the distances between the categories is not known -- for example small, medium, large. It is distinct from quantitative data.
In Chapter 4 of his book Designing Data Visualizations, Noah Ilinisky says:
Whether a visual property has a natural ordering is determined by whether the mechanics of our visual system and the “software” in our brains automatically—unintentionally—assign an order, or ranking, to different values of that property.
...
For example, position has a natural ordering; shape doesn’t. Length has a natural ordering; texture doesn’t (but pattern density does). Line thickness or weight has a natural ordering; line style (solid, dotted, dashed) doesn’t. Depending on the specifics of the visual property, its natural ordering may be well suited to representing quantitative differences (27, 33, 41), or ordinal differences (small, medium, large, enormous).
The image attached is also taken from his book. It suggests that line weight and color saturation (gradient) are intuitive ways to encode ordinal data.
Also shown in the image, a dotted/dashed line (line pattern) would not be a good choice for ordinal data.
You have decent ideas but there are setbacks: some people (7% of men) are colorblind; thick lines may hide thin lines; and a circus of dash/dotted lines may cause headaches.
Superimpose the flight value in a shape (ie circle) that is affixed to the flight path. Kind of like road maps shows the Route Number on a road.
If there is not enough room for a bunch of "dingbat numbers," then offer a toggle to turn them on/off by clicking on the path (or a master switch elsewhere).
Suggestion: Does it have to be done on the map or could you use a linked view? (This assumes this is a computer-based map and not a printed one)
As the above chart shows, the most powerful visual variable is position and on a map that's already used up by the end/start points of each route. Even if you use line thickness, colour etc, longer less-travelled routes are going to be more prominent than shorter, popular ones
So why not have the most popular routes in an ordered list in a sidebar to the left/right of the map? Have linked interactions so selecting / hovering on a route in one view highlights it in the other?
