Filter for ticks

What do you think about turning in addition `values` into somethink like filter of `x`? I mean the following possibility:

```
axis: {
x: {
tick: {
values: function (x) {
if(x.getDay() === 1) {
return x;
}
}
}
}
}
```

Also that would be great to have such a possibility for y axis as well, for example, when you need to show areachart with y-values from 80 to 100% range that would be much nicer to display every second tick and gridline (i.e. 80, 82, 84, ...)