PHPlot-6.2.0 added data borders for area and squaredarea plot types. If the Y values for each X in these plot types are not in decreasing order, the areas will overlap and successive areas will cover previous ones, as designed. However, the data borders (if on) are drawn at the end and outline what the areas would be without the overlap. So the borders don't correspond to the areas.
A first try at fixing this was to draw each border after the corresponding area fill, rather than all at the end. This works, but would require separate coding for the stacked variations of these plot types - which currently share the same code. So this was deferred until after 6.2.0.