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I use QgGIS 3.10 and this tutorial https://anitagraser.com/2019/05/04/flow-maps-in-qgis-no-plugins-needed/.

I am able to create lines of varying thickness and color depending on the weight of the flow in a virtual layer.

example1

I try to create arrows and eventually create curves

So I try to introduce the expression of the tutorial but without success. Note that the expression introduce a middle point in each line (but no curve and arrow)

example2

Note that when I use the arrow symbol, the width of the line is not configurable with the assistant.

example3

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  • Have you tried applying the geometry generator without classification? If so, what happens? – Erik May 5 '20 at 6:30
  • You are almost there! Click on Ligne simple (Simple Line in English version) and switch the Symbol layer type to Arrow. – Kazuhito May 5 '20 at 6:42
  • @Kazuhito Ok. I get curved lines but I cant change line size based on weight (see my new screen) – Wilcar May 5 '20 at 6:51
  • Maybe this thread can help you Adjusting flow map styles in QGIS 2. – Taras May 5 '20 at 6:52
  • @Erik the same. – Wilcar May 5 '20 at 6:52
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You are changing the line width of the wrong component (the 'fill' texture of the arrow).

You need to go to the Arrow section (Flèche) and change the arrow width, etc. there.

Have a close look at this screenshot from Anita's tutorial and see where she has used expressions/columns to modify arrow width on the right-hand side.

See also her post here on how to set up widths in flow maps.

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