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