Switch between two modes of displaying isolines, affecting [Isol-F] in the [Draw]-submenu. The major difference is how to draw the domain's background shape before drawing the isolines. The default is a "quick" mode where the program tries to draw and fill each subdomain as a single polygon. Since this may be incorrect if a subdomain had a hole inside (it would be filled as the background of the domain), you may switch to a "slow" mode where each small element of the domain is filled one by one.