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

# The maximum range in blocks to render holograms for players.
holograms-view-range: 48

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

# Section for placeholder settings.

  # Section for PlaceholderAPI settings.

    # Enable placeholders from PlaceholderAPI.
    enabled: true

    # The refresh rate in ticks for placeholders from PlaceholderAPI (default is 10 seconds, 20 ticks every second).
    default-refresh-interval-ticks: 200

    # Allow using the short format for PlaceholderAPI (i.e. %placeholder%).
    expand-short-format: true

# Section for the command "readimage" command.

  # Text to use to render solid pixels in an image.
  solid-pixel: █

  # Text to use to render transparent pixels in an image.
  transparent-pixel: '&7 ⎹ '

# Section for multi-server related placeholders (amount on online players, status and MOTD).

  # How frequently these 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

# Section for 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