The lower panel shows the sandbox surface with brightness representing height. The upper panel shows the cross sectional profile of the sandbox at the last row.
The width, $L_x$, of the sandbox is 256 and the length, $L_t$, is also 256. (However, since only even (odd) columns of sites are present on even (odd) row, there are only 128 sites on each row.) The driving wall is located at the bottom of the system which is period in the $x$ direction.
The surface plot is updated when the driving wall drop by half a unit from last update as well as right after a system spanning avalanche (avalanche that reaches the last row). The cross section plot is update for every system spanning avalanche.