Fix jagged lines sticking out from behind data point circles

[gsbernstein]

Goals ⚽

Prevent jagged lines from sticking out from behind the data points

Before	After

Implementation Details 🚧

I tried subclassing the LineChartRenderer to override drawLinear() but it's difficult because _xBounds and _lineSegments are internal protected.

Because this disconnects each line segment, I limited it only to the case where there's a data point circle covering the line join. If we want to be stricter about that check, we could also make sure that the circle size is greater than the line width, and that the color of the circle is fully opaque.

I also have a fix for the case where the disjoint isn't covered up, see [improve-jagged-lines-extra](https://github.com/gsbernstein/Charts/pull/1/files)

Testing Details 🔍

• demo app

[gsbernstein]

@pmairoldi — what is the best way to get a review for this? Should I update my commits? Do I need an issue?