Highlight specific hours in visual axis

Hi all,

I have a visual that shows how quickly an activity was performed by average (on hour).  The stakeholder has asked me to highlight specific hours on the axis so that a person that doesn't know what peak operating hours are can easily see them (these are static hours and not based on any data element).  Is there a way that I can highlight or add a visual to the visual axis to make these hours obvious (i.e. put a square outline around 8 - 16)?  When I try to insert a shape, it automatically goes behind the actual visual.