Add dynamic hotizontal line to graph

[Novack]

The CoinMarketCap dynamic (mouse following) horizontal line is pretty useful, and would be great to have that in CryptoTracker.

[ismaelestalayo]

Thanks for your suggestion!
I think it did cross my mind to implement that kind of horizontal line. However, I just checked the doc of the charting library used in my app and it only supports vertical lines.

(See https://docs.telerik.com/devtools/universal-windows-platform/controls/radchart/behaviors/charttrackballbehavior)