Update: I found the solution in the documentation, however I'm on version 7.1.2 and this approach doesn't work. Would there be any other way to decrease the height of the first 2 lines of the graph?
New versions of Highchart contain many new useful functionalities and fixes that we develop on an ongoing basis. Therefore, our support focuses mainly on the latest versions of the product. For this reason, the only thing I can recommend to you is to update Highcharts to the latest version.
Feel free to ask any further questions!
Best regards