How do I make major and minor grid lines have different thicknesses?
Here is a trick to get major and minor grid lines with different thicknesses. To do this, the major ticks must be shown inside or on both sides of the axes. By setting the Inside value for major tick marks to a large enough value in Axis Options, the tick marks will extend across the plot and look like major grid lines. You can then set the Tick Thickness to control the thickness of the major grids and the Grid Thickness to control the thickness of the minor grids.