Setting the line width like that should work. Try using style in the plot command? For transparency, I’m not sure what the alphas option does. You could try setting the line color to something with transparency, like so:
julia> x = rand(10)
julia> using Colors
julia> plot(x = x, Geom.density, style(line_width=3mm, default_color = RGBA(.1, .1, .9, .3)))
