Use this to set the format in which numbers on the axis will be drawn. E.g. you can use it to set a certain number of decimal values or prepend/append text to the numbers. Numbers will be drawn at all "major ticks" on the axis. Note that the "numberformat" is only used if the type of the axis is numerical (linear or at some later step even logarithmic), not if it's a textual axis (x-axis).