I like to create a bar chart to compare, e.g. the sales figures of product
abc, for the current year (from Jan to Dec) against the numbers for prior
year (from Jan to Dec). Basically with 2 bars (current and prior) side by
side for each month. The data can be on the same worksheet or in separate
worksheet for each year.

I am having a hard time to make it work. How can I define the data range
such that Excel will know it should be a comparison?

Any help will be very much appreciated.