MQTT cover history shows unavailable

I have an ESP8266-based garage door opener that I have integrated into HASS as a cover. Everything seems to work fine except that it does not show a history for the component.
Cover%20History

I have verified with mosquitto_sub that the availability topic has an available payload published when the ESP8266 boots.

Is that bar in the window saying that the history is unavailable or that the cover component itself is unavailable?

Here is my component description:
configuration.yaml:

cover: !include cover.yaml

cover.yaml

- platform: mqtt
  name: "Garage Door"
  command_topic: "home/garage/door/cmd"
  state_topic: "home/garage/door/state"
  set_position_topic: "home/garage/door/position"
  availability_topic: "home/garage/door/availability"
  qos: 0
  retain: true
  payload_open: "OPEN"
  payload_close: "CLOSE"
  payload_stop: "STOP"
  state_open: "open"
  state_closed: "closed"
  payload_available: "online"
  payload_not_available: "offline"
  optimistic: false