colorbar tick mark offset

[kswang1029]

The tick mark of the colorbar seems to have a small offset in the expected location. In this example, the pixel value varies from 0, 1, 2, ..., 8, 9 and NaN from left to right. If we configure the colorbar range from -0.5 to 9.5, we should expect to see the tick marks at 0, 1, 2, and so on are centered at corresponding color blocks. Apparently there is a small offset in each.

[kswang1029]

based on further test, this is due to the inconsistency of the color blocks. So I guess it is related to #1717