Reset y axis range to be dynamic

Hi,
Issue is about setting default range (undefined values).
If I set y max to 10 and then I'd like to set it back to be dynamic - I couldn't do it.
I resolved that issue by looking to `/src/api.axis.js`
There you check an input to be a value:

``` js
if (typeof max === 'object') {
if (isValue(max.x)) { config.axis_x_max = max.x; }
...
```

So I couldn't set `.axis.max({y: undefined})`. It's just not working.
And I saw one way that I can do reading your code:
when you check `if (typeof max === 'object')` and it is now (goes to `else`) there you don't do that check,
you just set an argument

``` js
config.axis_y_min = config.axis_y2_min = min;
```

So I used this type of assignment:
`.axis.min(undefined)`
and that worked.
So can you please provide a clear api that can reset range to be default(undefined)?