'Pinning' your arc lines
Apologies in advance, anyone reading this expecting to learn the complex art of drawing arc lines on a map in Tableau will be sorely disappointed. There are many great blog posts already written that explain the intricacies of the required table calcs and trig functions, check out those by Toan Hoang and Chris DeMartini, or alternatively you can create arc lines in seconds thanks to the magic of Tableau 2019.2.
Instead, this post shares a shamefully simple technique to give the illusion of your arc lines being ‘pinned’ to the background map. The idea for this visual technique came about when chatting through a viz idea with Sam Parsons. The UK State Visit of Donald Trump had just been announced and the news article referenced a list of State Visits was available via the royal family website… cue instant vizpiration!
I was experimenting with potential designs for using arc lines to map the State Visits of Queen Elizabeth II and Sam suggested the lines would have a greater impact if they appeared pinned to the viz. I immediately appreciated where he was coming from, when there are so many lines ending in close proximity, for example in Europe, arc lines can become crowded. It becomes difficult to distinguish the individual countries.
I started wondering how would be best to accomplish this grounding. Dual axis? Layered sheets? Then it hit me, why not simply show a text label for the line ends?!
I created a calculated field to identify the end of the line and then referenced a character of a filled circle:
If [Path Order] = 0 then '' else '●' END
Using this calculated field for the text label and centring the label vertically and horizontally, enabled the illusion of a circle to appear in the centre of the line end.
Finally, I coloured the circle white, to make it pop, and there we have it, an arc line with the appearance of the ends being pinned to the map. Nothing ground breaking, but I do think it makes the individual points more distinguishable and as such enhances the arc lines.
You can check out the final viz here.