Add hPlot to Highcharts

*** Work in progress ***

I have started to look a bit into adding a plot function to Highcharts - hPlot, and I create a pull request, although it is not ready to be implemented, to make it easier to comment and discuss on it.

Todo's:
- [ ] allow discrete axis values
- [ ] investigate on the behavior of NA-values
- [ ] add support for more chart types, e.g. pie chart
- [ ] Tooltip: replace "x"/"y" with labels