fix undefined data point handling

[mainettis]

Changed to find the first defined data item and use it to establish first position. Index can also be used to more accurately check if the data is an object type.
ref #34

var xy = false
if (dataset.data && typeof dataset.data[0] === 'object') xy = true

The above was replaced because the condition returns a boolean so we don't need an if statement to fill the boolean. Also, by this point dataset.data has already been checked and/or accessed, so we only need to check if a defined value is an object type.

Is there a reason not to terminate lines with ;?

[Makanz]

Thanks for your PR. Looks good!

Regarding not ending lines with ; is because a misconfiguration of prettier. I will fix that.