multiple touch support for line charts

**Describe the solution you'd like**
On apple stock app and robinhood a user can use two fingers to compare 2 different points on the chart. Having this would be great

No files, tests, or entry points are named. Start by locating the line-chart interaction and touch-handling code, then define how two fingers select and compare two points; done means the line chart supports that comparison behavior reliably.

Written by the indexing model from the issue text.