fillhandle=fill([vxlow,vxlow,vxhigh,vxhigh],[vyhigh,vylow,vylow,vyhigh],[.3,.4,0]);%creates the plot of a box. If you reverse the order of one of the sets of points, you get an hourglass shape
set(fillhandle,'FaceAlpha',.2,'EdgeAlpha',.2);%set edge color