Hi @Groumayo,
To achieve this, you first have to create a template with a dynamic named range for the input data that you use to generate the plots in the Graph results sheet. Follow Method 1 of https://support.microsoft.com/en-us/help/183446/how-to-use-defined-names-to-automatically-update-a-chart-range-in-exce to achieve that. You can also take a look at my template: template.xlsx (14.4 KB)
The trick in the workflow then is to create a copy of the template in a temporary directory and use an Excel Sheet Appender (XLS) to replace the Data sheet in the template. As a last step copy the file with the replaced sheet to the desired location. See 11863.knwf (35.4 KB) for an example that works together with the aforementioned template. You will have to adapt your paths in the two Table Creator nodes.
Together with the workflow that you have already shown, you should be able to tackle this.
Best,
Stefan