Sensitive skin. Dermatographia is where one's skin is sensitive to touch. Just scratching the skin may cause an angry red raised line. Cause is not usually known. Can be treated w/ antihistamine. For more info, type "dermatographia" in the "ask doctor box". Scroll to "see more results" and see what other doc have commented already on dermatographia.
Urticarial varient. Dermographism is a physical urticaria of uncertain etiology. It occurs when the skin is scratched and linear hives appear. They typically last a short time (minutes to hours) resolving with no changes in the skin. Treatment is aimed at breaking the itch/scratch cycle to stop vigorous scratching, and therefor, the hives.