A grid is associated with a primary and a secondary axis. The grid is displayed in context of the primary axis' label interval - meaning that a grid for an Y-axis displays a grid for every label on the y-axis (fx. a Image_Graph_Grid_Lines, which displays horizontal lines for every label on the y-axis from the x-axis minimum to the x-axis maximum). You should always add the grid as one of the first elements to the plotarea. This is due to the fact that elements are 'outputted' in the order they are added, i.e. if an grid is added *after* a chart, the grid will be displayed on top of the chart which is (probably) not desired.
Sets the starting and ending points of the grid, these defaults to 'min' and 'max' which signals that the grid should span the entire the perpendicular axis
Parameters:
mixed
$start
—
The starting value, use 'min' to start and "beginning" of the perpendicular axis
mixed
$end
—
The starting value, use 'min' to start and "end" of the perpendicular axis