Throw error if x and y are different sizes

This should throw an error instead of plotting something:
```
arphitgg(data.frame(), agg_aes(x=1:10,y=1:5))+agg_col()
```

Why? This is why:
```
arphitgg(data.frame(x=1:10,y=1:10,group=rep(1:2,5)), agg_aes(x=1:10,y=y,group=group))+agg_col()
```