Not sure if this applies here. But I think it's the usual approach to this issue.

The solution implies creating a cross-tab. Group by date (month) and use a formula as group name. The formula simply returns the date formatted as "MMM-yy". Use the same summarized field you want in the chart. Place this cross-tab in a section, and hide it.

Finally create your chart in another section using the cross-tab option. This way, the graph shows the X axis with the right format and the month order is also correct.