what does this graphic mean?

[-72-]

The white line always appears, that's the penetrated state indicator, it just looks somewhat odd in your case because it overlaps part of the larger minefield. It's otherwise working as it should as far as I can tell.

The newest official graphics, present in Moscow '42 and probably other updated titles as well, feature a diamond with a number in it to indicate obstacle/minefield strength, instead of the more conventional graphics of up to three mines. It makes things a bit more clear and it also prevents the white line from obscuring something.

Or this - boy I had not realised that it had been that long since I had played a PzC title.  .   .  (my bad).