Dataset before being written to:
 [  99  99  99  99  99  99  99]
 [  99  99  99  99  99  99  99]
 [  99  99  99  99  99  99  99]
 [  99  99  99  99  99  99  99]

Dataset after being written to:
 [   0  -1  -2  -3  -4  -5  -6]
 [   0   0   0   0   0   0   0]
 [   0   1   2   3   4   5   6]
 [   0   2   4   6   8  10  12]

Dataset after extension:
 [   0  -1  -2  -3  -4  -5  -6  99  99  99]
 [   0   0   0   0   0   0   0  99  99  99]
 [   0   1   2   3   4   5   6  99  99  99]
 [   0   2   4   6   8  10  12  99  99  99]
 [  99  99  99  99  99  99  99  99  99  99]
 [  99  99  99  99  99  99  99  99  99  99]