How can I stack a bar chart to compare traffic from different time periods?

Hi all.

I want to have a stacked bar chart to best visually compare traffic by month this year vs. last year. When I select stacked chart it does not give me an option to compare data to a time period.

Right now I have 2 separate regular bar charts for each year but am hoping someone knows how I can combine them into 1 stacked chart.

Thanks in advance.