The Zambia Revenue Authority has now started to publish aggregate statistical data on taxation and other issues. This is a good development, though the data needs to be more accessible in different formats (e.g. excel). It was painful just to put the chat below together - click to enlarge (real comparisons reveal very interest results - not shown below but will let others have a go). Hope it will be updated more regularly.
In case you are having trouble locating the tax data. The link is above and can also be found here (at the bottom of the page you will see options e.g Taxation, GDP. Click on one of those).