An example way of how to implement a Scripted GUI pie chart in Hearts of Iron 4.
A pie segment iconType equivalent to a 1/100 of the entire pie is created 100 times, each time with a 3.6 degree rotation, creating a full pie in the end. The texture contains frames - and which frame is shown is controlled by a 100 element pie_chart array, with each element corresponding to an iconType. The create_pie_chart scripted effect is used to create the pie_chart array, updating the pie chart. In this example, it is called daily.
The pie chart can be easily resized with scale argument in the .gui file. Changing the number of possible colors is also easy - one just needs to add/remove frames from the gfx/interface/pie_chart_segment.dds texture, and update the .gfx entry and the create_pie_chart scripted effect. This pie chart can be used to represent any values adding up to 100% - in this example, vanilla party popularities are used.
You may overlay a pie chart overlay onto this pie chart, to make it look nicer.
Feel free to use in your mods, but give credits to Yard1 (both in code, with comments; and on your download page).