I don't think you can change the size of the lines of the box plot; however, you can change the shadow behind the legend if you do as follows:
On the Presentation tab of the object, under legend, select Settings. Then make sure that Legend Style is set to Classic (your legend will end up looking like it does in my screen shot):
I hope this is helpful!
Thank you very much,
but this doesn't really solve the problem. The data points created with e.g. Aggr(If(Geo = 'Languedoc-Roussillon',Wert),[Handelspartner], Geo) from the beginning appear with a shadow in the legend, while they are displayed without a shadow in the graph.
This remains the same when changing the style of the legend.
Any other suggestions?