Hacky way:
xlims = (-0.05, 0.75)
plot(tab[:,1], tab[:,2]; xlabel="tax", xticks=[0.0, 0.2, 0.6, 0.7], xlims)
plot!([NaN], [NaN]; label="", yticks=[],
xticks=([0.1, 0.4, 0.65], ["A", "B", "C"]), tick_direction=:none,
grid=:none,
inset = (1, bbox(0.0, 0.0, 1.0, 1.0, :bottom, :right)),
subplot = 2,
bg_inside = nothing, xlims
)
gives
