I have met a problem which with drawing a histogram with contz option. As far as I understand it should draw contour lines filled in with color in between, but some times it draws an empty area filled with pad color only. The same situation presented in User_Guide_3_10.pdf on page 35. One can think that those areas do not contain any data but page 34 shows that there are some data. Expected picture should look like colz option but with areas separated by contour lines except of rectangles.
In fact, the CONTZ option is wrong in that particular case. This option draws a fill area for each level found. To draw a fill area a closed polygon is required. The current version closes the polygons by putting the last point equal to the first one. If all the contours are closed that works well, but if you have opened contours you may see wrong pictures like on page 35 of the manual. This has to be improved. With the new contouring algorithm I am working on (using Delaunay’s triangles) that will be easier tu separate opened contours and closed contours and therefore to produce a better drawing in such cases. Note that CONTZ has always been like that, it is not a “new bug”. In your case I would recommend to use an other option like COLZ for instance, or even better CONT4. CONT4 is based on the surface algorithm does not use polygons to draw the contours.