SetNumXTicks — Set the number of X tick marks
$plot->SetNumXTicks($nt)
SetNumXTicks sets the exact number of tick marks to
draw on the X axis.
You can use either this function or SetXTickIncrement
(but not both) to control the tick mark spacing.
If neither SetNumXTicks nor
SetXTickIncrement is used, the tick mark interval
is calculated as 1/10th of the X data range.