Suggestion: Background Lines

[SamMorrowDrums]

I briefly mentioned having a line in the background of a graph in #732

Sometimes, you want a complex 'x-line', rather than straight line. I'm using Scatter plots and curved lines to produce gaussian normal distributions. I would love it if (as in #732 I could hide the points from the bell curve without hiding the points on the scatter graph), but also it would be great to have a line in the background, meaning:

• No mousover tooltips or interactivity
• Not added to the Key
• Still affects the range of the x axis

[SamMorrowDrums]

Perhaps we could use 'line-background' as a type.

[masayuki0812]

Thanks for the suggestion. It looks not bad, but please mark it as enhancement at this moment.