If you are using Splunk Cloud, you won't be able to do that yourself. You could file a ticket and work with Support to get your changes in place. What are you trying to do that requires advanced XML instead of simple XML?
I need to overlay two values in one chart with a common X axis and a Y axis on either side
chart 1 - Area chart: No. of requests per server (X axis - server, Y axis on left side - no. of requests)
chart 2 - Area chart : Average Response time per server (X axis - server, Y axis on right side - average response time)