IMPORT currently provides the only means of getting asymmetric axis and data errors into ASTERIX applications. This is achieved using the LO/UPWIDTH and/or LO/UPERROR descriptors to describe the axis and data error bars respectively. See the "Asymmetric Errors" under the examples sub-topic for a demonstration of these descriptors.
An alternate pair of descriptors for settings lower and upper data errors is LOLIM/UPLIM. Here the data columns should specify the lower and upper error bounds, rather than the sizes of the errors themselves. IMPORT converts the bounds to errors using the value of the corresponding DATA element.