NOTE: The way that XPP computes nullclines (by computing zero contours of a two-variable function) means that the nullclines are composed of a series of small line segments. This means that if you try to plot them as a continuous curve, your plotting program will produce garbage. Thus, you should plot them as points and not draw line segments between them. The data file produced has 4 columns. The first two are the x nullcline and the second two are the y nullcline.
Data is as follows:
xnx1 xny1 ynx1 yny1 xnx2 xny2 ynx2 yny2 ...There are an even number of entries.
The x nullcline consists of segments (xnx1,xny1),(xnx2,xny2) between pairs of points. Similarly for the y nullcline.