I'm not sure if you can do this with the pre-made graphs in the web interface. What you can do is can use the Eco API to do what you want.
They do seem to work on a fresh local server just fine but return a value of 0 on my hosted server but it probably just has to be wiped.
One thing you could try is just getting the playerstats, for example: http://SERVER_IP:PORT/api/v1/analysis/playerstats/USERNAME?startDay=7&endDay=14
You will get a JSON encoded response back containing a list of the specified player's stats and one of those is Learn Skill which will say something like "Summary": "learned 78 skill points".
That might not be exactly what you're looking for. If you want individual skills you can query them with the dataset/get API operation by sending a JSON encoded dataset like so:
["Humans","USERNAME","Learn Skill", "Surivalist"]
["Humans","USERNAME","Learn Skill", "Surivalist","Basic Crafting"]
To view the API documentation you can go to http://SERVER_IP:PORT/swagger