Hey Jay
you should look at creating a new Data Expression on the Serial Chart settings. You could then create a featureset on the fly , splitting your MONTH/DAY field(s) and get the results you are looking for. (the sample link could give you some good ideas as well)
Hope that can help you. Good luck!
// Write an expression that returns a FeatureSet.
// Documentation: https://arcg.is/3c419TD
// Samples: https://arcg.is/38SEWWz
var fs = FeatureSetByPortalItem(Portal('https://arcgis.com/'), 'YOURITEMID', 0, ["CallDateTime","callYear"], false);
var drFilter = "callYear in (2020,2021) and Agency in('AFD')"
var drSel = Filter(fs,drFilter)
//return count(drSel)
var combinedDict = {
fields: [
{ name: "CallDateTime", type: "esriFieldTypeString" },
{ name: "callYear", type: "esriFieldTypeString" },
{ name: "callDay", type: "esriFieldTypeString" },
{ name: "callMonth", type: "esriFieldTypeString" },
{ name: "callMonthDay", type: "esriFieldTypeString" },
],
geometryType: "",
features: [],
};
var i = 0;
for (var d in drSel){
combinedDict.features[i] = {
attributes: {
CallDateTime: d["CallDateTime"],
callYear: d["callYear"],
callDay: Day(d["CallDateTime"]),
callMonth: ISOMonth(d["CallDateTime"]),
callMonthDay: ISOMonth(d["CallDateTime"])+'-'+Day(d["CallDateTime"])
},
};
i++;
//return d["CallDateTime"] + " " + ISOMonth(d["CallDateTime"]) + " "+ Day(d["CallDateTime"])
}
//return combinedDict;
return FeatureSet(Text(combinedDict));