jQuery flot and y-axis zones

I'm developing a graph for a client using jQuery flot. The graph will show the daily air quality indices for the summer months. There is, however, an exceedence level which I'd like to highlight on the graph. What first comes to mind is a reddish background for the exceedance zone across the x-axis, beginning at the y-axis exceedance and going up. Does anyone know of a way of doing this with the flot code?