Something like this?
Note that my scripted field is a String, not a Date, which is created from an existing Date field. I'm using the Kibana sample E-Commerce data as a reference. The script is as follows (change order_date to timestamp in your case).
ZonedDateTime input = doc['order_date'].value;
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MMMM yyyy");
String output = input.format(formatter);
return output;
This is how the data shows up in a Lens visualization, when I drag the scripted field to workspace to use it as the horizontal axis field:
