By default, the number of grid lines is always equal to the number of visible scale labels,
no matter whether a scale is automatic or not.
To specify what grid lines are visible or not, use the parameter
graphsetx_gridvisiblelines. This parameter takes a sequence of integer numbers,
each number corresponding to an index ranging from 0 to the maximum number
of lines displayed minus 1.
If chart is vertical the indexes increase from top to bottom,
and if chart is horizontal, from left to right. For instance, if a vertical chart is displaying
a scale with 10 tick marks, and only the first, fifth and tenth grid lines from top to bottom
must be displayed, the values 0, 4 and 9 has to be passed to the parameter above.
The following example only displays the second and third grid lines, from top to bottom: