if _max equal to _min cause crash 

```
- (CGFloat)minVerticalBound
{
    return MIN(_min, 0);
}

- (CGFloat)maxVerticalBound
{
    return MAX(_max, 0);
}

```

if _max =0 and _min = 0 then  minVerticalBound and  maxVerticalBound will be 0

```
CGFloat scale = _axisHeight / (maxBound - minBound);
```

this will cause a crash
