I didn't test this script (I don't have a BACnet device with a trend log on hand), but I adapted it from this thread to print the LogRecord objects in the 'logBuffer' property of Trend Log object number 1: https://forum.infiniteautomation.com/topic/3117/bacnet-scheduler

//Get the local device config from a configured bacnet data source.
//Edit DS XID
var localDeviceConfig = com.serotonin.m2m2.db.dao.DataSourceDao.instance.getByXid("DS_354e2188-4867-431b-9051-bca39b011ed7").getLocalDeviceConfig();
localDeviceConfig = com.serotonin.ma.bacnet.device.LocalDeviceDwr.getLocalDevice(localDeviceConfig);
var listener = new com.serotonin.ma.bacnet.ScriptableDeviceEventListener();

listener.registerListenerExceptionHandler( function(excp) { print("Listener Exception"); });
listener.registerIAmReceivedHandler( function(removeDevice) {print("Remote device");});
listener.registerAllowPropertyWriteHandler( function(from, obj, pv) {print("Allow property write");});
listener.registerPropertyWrittenHandler( function(from, obj, pv) {print("Property written");});
listener.registerIHaveReceivedHandler( function(removeDevice, removeObject) {print("I have received");});
listener.registerCovNotificationReceivedHandler( function(subIdentifier, initDeviceIdentifier, monitoredObjectIdentifier,
	timeRemaining, listOfValues) {print("COV notification");});
listener.registerEventNotificationReceivedHandler( function(processIdentifier, initDeviceIdentifier, eventObjectIdentifier, timestamp,
	notificationClass, priority, eventType, messageText, notifyType, ackRequired, fromState, toState, eventValues) 
	{print("Register event notification received");});
listener.registerTextMessageReceivedHandler( function(textMessageSourceDevice, messageClass, messagePriority, message) 
    {print("Text message received");});
listener.registerSynchronizeTimeHandler( function(from, dateTime, utc) {print("Sync time received");});
listener.registerRequestReceivedHandler( function(from, service) {print("Request received");});

var localDevice = com.serotonin.ma.bacnet.device.LocalDeviceFactory.getLocalDevice(localDeviceConfig, listener);
//print(localDevice);

try {
    //Create the read request... See com.serotonin.bacnet4j.type.enumerated.ObjectType for object types
    var objectIdentifier = new com.serotonin.bacnet4j.type.primitive.ObjectIdentifier(com.serotonin.bacnet4j.type.enumerated.ObjectType.trend    Log, 1); //EDIT: object instance number
    var propertyIdentifier = com.serotonin.bacnet4j.type.enumerated.PropertyIdentifier.logBuffer;
    var readRequest = new com.serotonin.bacnet4j.service.confirmed.ReadPropertyRequest(objectIdentifier, propertyIdentifier);

    //Get the remote device and send the request, edit the remote device number
    var remoteDevice = localDevice.getRemoteDeviceBlocking(123);

    var result = localDevice.send(remoteDevice, readRequest).get();
	//result should be a read service ack that we can hopefully get a com.serotonin.bacnet4j.type.constructed.BACnetArray of com.serotonin.bacnet4j.type.constructed.LogRecord from
	var logRecords = result.getValue(); //now iterate this list and do what
	for(var k = 0; k < logRecords.size(); k+=1) {
		print(logRecords.get(k).toString());
	}
} finally {
    com.serotonin.ma.bacnet.device.LocalDeviceFactory.releaseLocalDevice(localDevice, listener);
}

You may need to edit the data source xid, object instance number, and remote device id, as noted in the comments.

Mango does not support polling Trend Log points as one would an analog value. You can probably read any property on those objects, either through the API (takes some effort) or through a scripting data source (get the local device, figure out from the BACnet4J code how to send read property requests)..

What property would you be trying to read from a trend log object? They do not have a 'presentValue' property like the other readable points do.

I started to mess with bacnet now and I'm kind of lost, but thank you !!

Would not it be possible to pull the value of a TrendLog type variable?