Manually place ticks at certain positions

Ticks can be set manually by passing tick instances to the `manualticks`
argument of the axis instance as shown at the right axis in this example. ...

As you can see, the manual ticks are combined with the automatically created
ticks. You could disable any automatic tick creation by setting the `parter`
argument to None. But what's much more interesting is, that the manually and
automatically created ticks are not just merged, but their combination is used
for rating the resulting partition. Therefore, the additional manual ticks lead
to a different set of automatically created ticks in this example.
