[Chart] A bunch of features and fixes.

[malcolm.wallace]

> Thanks - I'll work my way though looking at these. I've started with the sparkline stuff. It looks fine (and useful). Two comments:
>
> 1) field names in the SparkLine and SparkOption structures. As you would have no doubt noticed, I add a prefix to all public structure to avoid naming overlaps. 
 
Yes, that is my usual practice as well.  I think I just copied the data structure from the existing sparklines package on hackage, which is why I did not remember to change this.  Feel free to adjust to taste.

I did also wonder whether it might make sense to release the sparklines as a separate package, depending on Chart?

Regards,
    Malcolm
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