(No version information available, might only be in SVN)
Matrices are used throughout cairo to convert between different coordinate spaces.
$xx = 1.0
[, float $yx = 0.0
[, float $xy = 0.0
[, float $yy = 1.0
[, float $x0 = 0.0
[, float $y0 = 0.0
]]]]]] )