(solved) Data field not being displayed in Thingspeak channel

I have implemented a webhhook on TTN for Thingspeak. I can see on the Thingspeak channel page that entries are being pushed (Under the Channel Stats heading in Thingspeak, I can see that the Last Entry time stamp corresponds to the live data timestamp on TTN. However no data is shown on the Thingspeak line graph. For my uplink decoder in TTN I have tried just a simple:

function decodeUplink(input) {
  return {
    data: {
      field1: 21
    warnings: [],
    errors: []

Should this not push the value of 21 to my channel? ( can do an update in a browser with an HTTP request with: &field1=21 in the GET request)
Solved: - I had my decoder in Payload Formatters (uplink) instead of the end device specific payload
thanks all