pango-cairo-error-underline-path (cr x y width height)

cr -- a cairo-t context
x -- a double float x coordinate of one corner of the rectangle
y -- a double float < coordinate of one corner of the rectangle
width -- a double float non-negative width of the rectangle
height -- a double float non-negative height of the rectangle

Add a squiggly line to the current path in the specified Cairo context that approximately covers the given rectangle in the style of an underline used to indicate a spelling error. The width of the underline is rounded to an integer number of up/down segments and the resulting rectangle is centered in the original rectangle.