This app shows how the EU's CO2 emissions would develop under the Paris treaty. It can be found under http://eu.climate-calculator.info/.
The global emission budget before reaching a temperature threshold is a scientific question. How this amount is distributed to countries, however, is a political question. The tool addresses this topic by letting the user choose between two allocation mechanisms: allocation by population share and allocation by emission share of the EU.
Once the starting point (emissions 2019), the emission budget between 2020 and 2100 and the maximum possible negative emissions are determined, a free parameter has to be determined by means of a non-linear optimization to be able to plot the emission pathway.
The full mathematical methodology of the so called "Extended Smooth Pathway Model" can be found under https://klima-retten.info/downloads.html.
Screenshot of the app:
The app is written in R and is largely based on the shiny package.