Since we want some discrete horizontal grid lines we might think that it is enough to do the normal

1

$graph->ygrid->Show();

However, doing that will not show any grid lines. The reason is hat by default the grid lines are drawn at the bottom of the plot. Since we have filled area plots covering 100% of the pot area no grid lines would show. In order to change this we need to move the grid line to the front with a call to the method

using the argument DEPTH_FRONT. The rest of the grid line formatting is just basic style and color modification to make the grid visible but just barely.