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Considering the x-axis uses a decimal point and the y-axis a decimal comma, my guess is your df[:, 5] is parsed as a Vector{String}. If so, you could convert it to a Vector{Float64} using parse.(Float64, replace.(df[:, 5], ',' => ".")), which will first replace every (decimal) comma by a (decimal) point, and then convert all these Strings to Float64.
Once your y-values are floats, the y-axis should behave as you’d expect.