I've seen one example in which you would have to click on a point and the software would plot the line that goes through that point. Of course, in that case the density of lines may or may not represent the flux density, depending on where you click.
Off the top of my head, I suppose you could start at the point of largest magnetic field (call the value B) and plot the field line going through there. Then move some reasonable distance R to one side (perpendicular to the magnetic field vector); call the magnetic field value there B' and plot another field line going through that point. Then move over again by the distance R'= RB/B' and plot another line, etc. until you reach the edge of the graph or something.
If you don't wind up getting a good answer here, you might have better luck asking on a computer programming Q&A site (such as stackoverflow.com).