The legend or legend_position attribute is described here:
Show the legend ?
Can also be a (x,y) tuple or Symbol (legend position) or angle (angle,inout) tuple.
Bottom left corner of legend is placed at (x,y).
Choose from (:none,:best,:inline,:inside,:legend) or any valid combination of:(outer ?)(top/bottom ?)(right/left ?),
i.e.::top,:topright,:outerleft,:outerbottomright…
(note: only some may be supported in each backend).
on the page Subplot Attributes · Plots
However, if this attribute is not supported for the plotlyjs backend in Plots.jl, to get full control of the legend position you may need to use PlotlyJS.jl and use layout settings, for example, Layout in Julia