Add feature to add dashed border around a bar

[SayuruSandaru]

Please enable to add a feature to add dashed border to a bar in the barchart. This can useful when the user need to present a predicted data in the future. Below is a sample graph that I took from an app.

[Dartek12]

Related issue: #1194

[k0psutin]

Hey!

I have been working with Flutter for about a year now. I've been using fl_chart in few of my projects. I really like this project so I would like to contribute to the project and I would like to do this feature. :)

I was thinking that, we could add dashArray parameter to BarChartRodData and use that together with drawPath.

I'll read up on the contributing guidelines on the way. :)

[k0psutin]

@imaNNeo Is it intentional that, the border is drawn before drawStackItemBorderStroke? If the border is drawn before drawStackItemBorderStroke, then the BarChartRodStackItem will eat away some of the border, see pic:

It's only noticeable on really thick borders. On the other hand, if we draw the border last, we will get:

[imaNNeo]

Landed in 0.65.0
Check it out!

[imaNNeo]

@imaNNeo Is it intentional that, the border is drawn before drawStackItemBorderStroke? If the border is drawn before drawStackItemBorderStroke, then the BarChartRodStackItem will eat away some of the border, see pic:

I can't recall the intention behind that