xyscan can handle tilted plots although the accuracy will be somewhat degraded. It is very important that the markers are placed as accurately as possible. Here's an example of a tilted plot:
The exact position of the markers on the canvas allows the program to determine the tilt and take it into account. Note that xyscan cannot handle distorted plots of any kind other than tilts. Also keep in mind that the precision is severely deteriorated when scanning from tilted graphs, especially for the error bars. It is strongly recommended to correct the tilt beforehand using one of the many graphics manipulation programs available.