Text halo (outline) properties shared by geomText() and geomTextRepel().
Color of the halo around the text. Setting haloColor alone has no visible effect unless haloWidth > 0. When omitted, the panel background color is used (falling back to the plot background color when the panel draws no rectangle).
haloColor
haloWidth > 0
Width of the halo (outline) around the text. A halo is rendered only when haloWidth > 0.