SetXTitleColor — Set the color of the X Title
$plot->SetXTitleColor($color
)
Use this function if you want the X title to have a different color than the main title. See Section 3.7.1, “Titles” for more about plot titles.
By default, the X title defaults to use the same color as the main plot title. The main plot title color is set with SetTitleColor, and it defaults to black.