Hi Lars, ChartJS widget is a great and effective widget, but it requires a little bit of background info. As first download the Github Mendix project which can be found in the 'test' folder and try to run this one in Mendix 6.9.1 for example. That Mendix project contains all kinds of examples charting with data.
The first thing you should know with this widget the datastructure which is explained in the Github project. Extra info is that each dataseries should contain the exact amount of datapoints. ChartJS isn't that clever to figure out which datapoints are missing and adding them to the chart.
I understand that the challenge for you is: how to transform your datastructure to the datastructure of ChartJS. What I normally do related with dates is transforming a date to an integer for the 'xsortingvalue' according the format: 'yyyyMMdd'.
I hope this will help you the right direction.