Ability to create a complete new chart when the data updates 

The current `updateData` methods lacks the ability to remove or add new data. (It seems like there isn't any good solution for ChartJS in the library itself). 

#10 has the same problem. 

This merge will fix the problem when adding `destroy-on-update` to the `<canvas>` element. It simply destroys and recreates the chart. 

This is just a temporary workaround, which works, until a better solution is found. 
Setting variables make the code clearer but it wouldn't guard vs `oldVal` not existing for example.