I have observed your requirements and not able to completely understand which portion of chart background you are trying to change. Can you please share the generated presentation along with information that for which chart portion you need to change the background. I will investigate the issue further on our end to help you out.
And let me know what you see. Notice the last parameter. I get a gray background in the plot area when set to false (the default is true - which includes the sample data).
I have shared the sample code with default sample data. The following line is actually related to fulfillment of your requirement related to clearing grey background. You can use any data for chart as per your requirement.