I answered a similar question elsewhere, Re: Shade feasible region in chart.

Given several intersecting lines (constraints), shade the region that falls below all of the lines. I added a hidden series every N units (several tens of points along the X axis) and added vertical error bars to produce the shading.

FillBelowErrorBars2d.png

Instructions are at that other link, but I'll recreate them here if anyone wants me to.