Grid Interval by years, months, weeks, doesn't work (only work by days)

Grid interval by years doesn't work. The following code

```
Interval = 1.0
IntervalOffsetType = DateTimeIntervalType.Years
```

will actually interval by 1 day.

I read the source code, I found out that the coresponding relationship should be:

```
Interval IntervalType
IntervalOffset InterOffsetType
```

(IntervalOffset means to leave the first grid from zero-offset blank for `offset` spaces.)

So probobaily this problem is caused by :

`IntervalType` _missing_ in `type Grid`.

I found this by check the type LabelStyle, the label `interval IntervalType` works well.