allow scale expand

currently there's a default 10%, for scales that are cont and the low is not 0.0. this works but make some plots ugly.
allow user to expand as needed

scale_x_continuous(expand = expansion(mult = c(m1, m2), add = c(a1, a2)))
scale_y_continuous(expand = expansion(mult = c(m1, m2), add = c(a1, a2)))