Piwik already offers several different Charts to visualize data. Neither Bar Charts nor Pie Charts allow you to visualize the evolution of percentages over time.
Imagine you start a campaign for your website in a single country and you want to see if over time more people from that country visit your website. Right now, the only thing you can do, is select two dates and check the different numbers.
I suggest to add a new Chart Type: 100% stacked area chart, which would allow users to see how their website performs over time in different countries.
This chart is not only limited to countries. E.g. the IPv6Usage plugin I am building right now could use this chart to show how the IPv6 reach of a website increases over time (a line chart might be misleading here).
Examples of 100% stacked area charts: http://ipv6-test.com/stats/country/DE
Thanks for the suggestion. I think stacked chart are quite similar to Evolution graphs. I don't think it would a lot of value to the UI if we supported this new graph type so moving to feature requests. I think it would be great to improve the current graphs since there are so many ways we could make them more usable :) see: #3110