What do you mean exactly by "randomly distribute"?
You can change the "foreground" color by dragging the legend names to a different order (so put "5 ou moins" at the top, etc.) That at least would put the colors in the natural order.
But you only have 4 groups, and each group = one color, so "random" isn't going to make a lot of difference.
You could also play with the transparency of the colors, and/or add a border around each data point, to show where the overlaps are.
When you look at your results, what would you prefer to see to convey your intended meaning?
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I found what I was seeking. Many thanks to Jade Le Van & Florian Ramseger for their helpful talk on dense data. Is Your Data Dense? - YouTube
It as as simple as moving the details pill in the marks shelve to the top, above the color pill. If necessary create a field with a random formula.