I have a problem with the borders vs no borders example. After applying the borders, Edge looks like it has less visitors than Safari. This is because the border is white, as the background.
Borders did make the graph look better though. It would likely be better for situations where each bar has the same number of sections. Otherwise, making the white border overlap the graph itself (so it doesn't increase the size of the bars) could keep it from being misleading. Of course then small sections could get completely covered by the border...