I want to change the font size of the x-axis tick label in xml, but I haven't been able to fix this by using chart.fontSize
This requires a custom/advanced xml dashboard or default css hack. See these:
https://answers.splunk.com/answers/203373/in-splunk-61x-how-do-i-change-the-font-size-of-a-s-1.html
https://answers.splunk.com/answers/59062/changing-font-and-color-in-the-body-of-a-simple-table.html
https://answers.splunk.com/answers/277847/how-does-one-change-results-table-font-size-in-a-d.html
This is quite the popular topic. Sounds like a feature request to me...