Add a visualisation to the language family visualisation tool that shows the development of a language's dictionary over time (i.e., how many stems starting when it was created up to the present). There is a script that generates the data for this and some sample data. This graph should go in the bottom-left pane of the visualisation; if another graph is already there when you work on this task, make a switch to toggle between the two visualisations.
For further information and guidance on this task, you are encouraged to come to our IRC channel.