boxplot() and xticklabels

There's a bug with the xticklabels using boxplot:

``` python
a = [[1,2,3,4],[1,2,3,4],[1,2,3,4]]
clf()
boxplot(a, False, 'k', positions=[1000,1500,2000], widths=50)
xlim(750,2250)
draw()
```

With master branch, I see the xticklabels show as "1", "2", "3"
with 1.3.x I see "1000", "1500", "2000" as expected.

[tac edited for markup]
