There are lots of little limitations like this in FileMaker's charting. What is nice is that FileMaker charting is easy. But specific formatting and tweaking of the charts is often not possible. For this reason, I often use Google Charts API and a Web Viewer to display them and you have a lot more choices to work with. It certainly gives you many many more options than FileMaker's charting, but it also is a lot more complicated. You need to know HTML and character manipulation in FileMaker very well to make the HTML that will pull in the chart the way you want. There probably are some other good charting APIs out there that are good too, but Google is used a lot and well documented.
How do you use 4 fields in x axis?
I assume something like
List ( Count ; low ; avg ; high )
with Data source as "current record", then you can use "Text" for Data Type then use any calculation on it.