Please notice, I am able to reproduce the issue as you mentioned. I tried to minimize the plot area but to no avail. I used the following sample code with your template file but the plot area is not changed.
Workbook workbook = new Workbook("f:\\files\\SampleExcel.xls");
Worksheet worksheet = workbook.getWorksheets().get(0);
Chart chart = worksheet.getCharts().get(0);
I have logged a ticket with an id “CELLSJAVA-44812” for your issue. We will look into it soon.
Once we have an update on it, we will let you know.