Improve curve differentiation

[meguiraun]

both for the live view as well as for the downloaded png

[johanfforsberg]

As I mentioned in #17, some possibilities could be opened if we change the way the plots are generated. For example, we could highlight the curve that the user currently hovers over, and make it fast to hide/show curves that are obscuring the view.

Another feature that would help is to present a "tooltip" or similar that shows the name of the attribute for the curve currently closest to the mouse pointer. In fact, this feature was originally in the application, but it was sort of hacky and didn't work reliably so I removed it. But the necessary information is still available as a JSON blob included with the plot. With some work this could be a helpful feature.