The 6 mm thickness shows up as two edges + four grid lines. In a similarly odd way, the 40 mm width is 8 major grid lines * 5 ticks each. It includes the left edge, but not the right.
:shrug: Confusing artifacts of the graphing program -- I should have turned the grid off, but I was using 'em to make sure the aspect ratio was correct.
The isotherms are integers. There's only seven because the cold, far corners are at 304.5 with the
h I was using. When I tried to get it to label the contours, the labels overlapped each other and the graph.
Anyway, that model is a day old and already obsolete.
Here a different set-up, with two sets of isotherms corresponding to two different convection coefficients. I'm curious to whether the differing shapes are real or model inaccuracy.
