Add lines with dashes to chart

[nickromano]

Thank you for the great charting library! One feature I needed was the ability to add dashed lines.

The parameter is stroke_dasharray https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-dasharray. But I would be open to naming it something a little easier to understand instead like dashed=True and set a default (2%?). Let me know what you think!

Thank you, Nick