fix: change binField to use min provided by bins

[kwonoh]

If I understood binField and bins functions correctly, min provided by bins should be used for the output of binField since nice=true changes the min.

This PR replaces min to b.min

[kwonoh]

Repro: https://observablehq.com/d/137aafc8775f84fb

The min of values is 0.3 and the max of values is 9. However, the generated histogram looks like there is a value between 9.3 and 10.3.

I believe this is because of the below reason.

base and alpha in binField function are computed with b.min which is the "nicer" min returned by bins function.

However, the returning value of binField is computed with the original min.

Thus, I think the min in line 41 should be changed to b.min.

[jheer]

Good catch, thanks for the PR!