I found a number of links to details on how to customize graphics objects with the graphicname.property format (I googled idl graphics axes properties):
https://envi.geoscene.cn/help/Subsystems/idl/Content/GuideMe/Axes.htm
https://www.nv5geospatialsoftware.com › docs › AxesInGraphics.html
https://www.nv5geospatialsoftware.com › docs › Axes.html
https://envi.geoscene.cn/help/Subsystems/idl/Content/GuideMe/Axes.htm