A technique I sometimes use to indicate direction on a dot plot is connecting the dots with a triangle as an svg path and add a gradient to it. I think it stresses the direction more than a simple arrow because the direction is present on the whole shape (and in the colors), while arrows have only arrowheads to show the direction.
This is a remake of a remake by Jon Schwabish, with data from PEW. As Jon and others suggested: this type of dot plot might be more suited for data that really shows an evolution (fe values for 2015 with values for 2016) then for data that shows a comparison (Trump vs Obama).