    Will draw tick marks pointing in the downward direction every *gap* distance
    along the innermost closed contours only. User may choose to tick only local
    highs or local lows by appending the directives **h** or **l**, respectively.
    This behavior can be further changed with these modifiers:

    - **+a** - Tick all closed contours, not just the innermost.
    - **+d** - Append *gap*\ [/*length*] to set the gap between ticks and optionally the tick
      mark *length* (append units as **c**, **i**, or **p**) or use defaults
      [15\ **p**/3\ **p**].
    - **+l** - Append *labels* to set how to annotate the centers of closed innermost contours
      (i.e., the local lows and highs). If no *labels* is appended we use **-**
      and **+** as the labels. Appending exactly two characters, e.g., **+l**\ *LH*,
      will plot the two characters (here, L and H) as labels. For more elaborate
      labels, separate the low and high label strings with a comma (e.g.,
      **+l**\ *lo*,\ *hi*). If a file is given by |-C| and |-T| is set,
      then only contours marked with upper case C or A will have tick marks
      [and annotations]. **Note**: The labeling of local highs and lows may plot sometimes
      outside the innermost contour since only the mean value of the contour coordinates
      is used to position the label.
  