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