Skewed right-most tick on chart when resolution > 1 and data above maximum point not available

[kristovatlas]

For example, if we try to chart blocks 0 to 50 with a resolution of 10 and no data is available for blocks 51+, we'll get:

0-9: ten blocks of data divided by ten blocks
10-19: ""
20-29: ""
30-39: ""
40-49: ""
50-50: one block of data divided by ten blocks

for the last tick, divide by at most the number of blocks we actually have data for and not by block_resolution.

[kristovatlas]

I thought I had already addressed this in db.Database.get_blame_stats_for_block_span_and_resolution, so it's possible I'm not correctly assessing the root cause.