add animation for data lines and points

[dhf]

• Displayed points for series in the graph (commit from pryaniq, see https://github.com/pryaniq/SwiftChart/commit/5550bf1f79bf8f8827f96b941194ccad2da7270a)
• Animate line drawing and point drawing. This can be configured via the new Struct ChartAnimation.
  Howto:
  chart.animation.enabled = true/false
  chart.animation.duration = 1

Animation is disabled by default. Also updated the sample project.

[gpbl]

Hi @dhf thanks a lot for you PR!

The idea of supporting animations is tracked here (see also this PR) but the reason it's not implemented yet is that it is more difficult to implement as I thought.

Look for example how the animation works with the others examples:

With a single line chart it seems performing well, but

• the animation's duration depends on the number of values, e.g. if there are less values it seems way too slow.
• I expect the animation of a series with less values to end before the animation of a series with more values
• the lines above and below the x-axis are rendered separately (so we can give a different color), hence the animation looks weird
• background of filled series is not animated

I think we should find another way, maybe using a mask over the chart. I even would prefer to have the dots in a different PR, since it is another kind of enhancement. @pryaniq PR needs to be reviewed a bit, see discussion.