The configuration is located in /plugins/HolographicDisplays/config.yml.

This is the default configuration with descriptions:

# How much space there should be between the lines of holograms.
space-between-lines: 0.02

# This ensures holograms stay in the correct position when moved.
# Can be disabled if there are no external plugin using holograms that move,
# for example chat bubbles above the head.
precise-hologram-movement: true

# Display a prompt after editing a hologram to quicky execute more commands.
quick-edit-commands: true

# This section is related to the command /hd readimage.
# It controls how the image is rendered.
# By default a full block is used for the solid part of the image and
# a spaced, thin, vertical line is used for the transparent part of the image.
# They have the same width on the default texture pack.
# You can display the transparency with two spaces, but the image won't be perfectly aligned.
# You can use symbols from symbols.yml.
  symbol: "[x]"
    space: " [|] "
    color: "&7"

# This section is related to some placeholders that can display
# the amount on online players, the status or the MOTD of a server.

  # How frequently should placeholders be updated.
  refresh-seconds: 3

  # Enable if you use RedisBungee.
  use-RedisBungee: false

  # Used to display more information other than the amount of players in
  # a BungeeCord server, such as its status or MOTD.
  # The pinger is also able to query external servers.

    # Enable this if you want the features of the pinger.
    # Uses "refresh-seconds" as refresh interval.
    enable: false

    # A list of servers to ping, formatted as "<name>: <IP>:<port>".
    - "hub:"
    - "survival:"
    - "minigames:"

    # Timeout in milliseconds before assuming a server is offline.
    timeout: 500

    # The MOTD to display for offline servers.
    offline-motd: "&cOffline, couldn't get the MOTD."

    # The text to display for online and offline servers.
      online: "&aOnline"
      offline: "&cOffline"

    # Remove the leading and trailing whitespace from the MOTD of servers.
    motd-remove-leading-trailing-spaces: true

# Time format and timezone.
# Custom formats:
  format: H:mm
  zone: GMT+1

# Enable or disable update notifications.
update-notification: true

# Display debug information on the console. Keep it disabled.
debug: false