Duration is just by subtracting send datetime from receive datetime. But which kind of chart you'd like to have? You have at least few employees, many emails for each, Is that distribution of response time by dates, or min/max, or average or what?
As variant, if you have duration of each response in one column against employees names in another column, you may build PivotTable on this range aggregating duration as min, max and average. On the top of this PivotTable you may add PivotChart for visualization of this information. Something like this