Ticklen Keyword
Used With Routines:  AXIS, BAR3D,  BOXPLOT,  CONTOUR,  CONTOURFS,  CONTOUR2,  OPLOT,  PLOTSHADE_SURF,  SHADE_SURF_IRR,  SURFACE
Corresponding System Variable:  !P.Ticklen
Controls the length of the axis tick marks, expressed as a fraction of the window size. The default value is 0.02. Ticklen of 0.5 produces a grid, while a negative Ticklen makes tick marks that extend outside the plot region, rather than inwards.
Example
To produce outward-going tick marks of the normal length:
PLOT, X, Y, Ticklen = -0.02
To provide a new default tick length, set the system variable !P.Ticklen.