When the user selects the Delta mode for charts, the y-axis range typically includes both negative and positive numbers. generateTicks() handles this case specially in order to draw a horizontal line at zero in order to make it easier to see when the timeseries improved or regressed versus the starting point (e.g. previous milestone).
generateTicks() currently always displays 2 ticks above and below zero. However, this can look bad when zero is close to the top or bottom of the range. This function should continue to display 2 ticks both above and below zero when zero is close to the middle of the range, but change to display 0-4 ticks above/below zero otherwise.