I have been following this, but I didn’t have time to comment.
You asked about the. json file. Simplyfied, it’s a file that holds a (JS) object for data sharing.
{{date:"2018-01-01", litre:500},{date:"2018-01-02", litre:498},{date:"2018-01-03", litre:495}}
Probably comparable to what you did with your txt file.
For a working solution, that would actually be enough. Grab the json, handle the object, print the result with highcharts.
To me, the SQL db seems a bit “heavy weight” for such a simple use case, but it’s solid if you can make it work. Just my opinion.