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