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.
$tiDesired tick interval, in world coordinates. If the value is omitted or an empty string, the default behavior is restored.
If neither SetXTickIncrement nor
SetNumXTicks is used, the tick mark interval
is calculated as 1/10th of the X data range.