The usual techniques in thematic mapping (area colouring, symbols centered on a territory) are well suited for single-localized data, but are more complex to implement for bi-localized data. 
"Origin-destination" data is potentially much bigger, resulting in longer loading and processing times. Their mapping is difficult to reduce to a simple and legible visualization: arrows overlap or cross each other, zooming in on the map hide part of the data (origin or destination or both), orientation can be difficult to guess.
Various techniques will be described that can help web flow maps to get more meaningful:
- a specific choice of shapes, orientation signs, and colour palettes
- tooltips delivering relevant information on hovering each symbol
- interactive filtering techniques
- alternative representations to the drawing of arrows from A to B
- supplementing them with summary charts that reflect global changes over time

We may also discuss some techniques to ensure good speed and rendering quality on all browsers and devices.

Eric Mauvière is the founder of Emc3 (Toulouse, France) and creator of the Géoclip software, a generator of statistical and cartographic web observatories. Statistician-economist trained at INSEE, the french statistical institute, he has been working for 15 years with his team of 5 engineers to share statistical data on the web through interactive, fast and legible data-visualizations. The representation of flows is a fairly new field for Géoclip. This workshop is a good opportunity to share and gather good ideas!