add axref and ayref

[GHvW]

Hello!

summary:
I added the axref and ayref fields for the Plotly Annotations. I'm sorry if this wasn't the right way to address an issue for this repo. If it isn't, please let me know what I should do and I will be happy to go through the proper channels!

long:
I was using the new and amazing .NET Interactive Notebooks extension in VSCode and was attempting to use annotations as a pseudo "vector". I was struggling with ax and ay being tied to pixel values. I noticed the JS version has an ayref and axref but XPlot doesn't. Looking through the code it seemed like all of the hard stuff would be handled for me if I just added axref and ayref to the Annotation.fs file.

It looks like it works. A quick modification to one of the tests/scripts/XPlot.Plotly/ fsx scripts generated a nice "vector" for me:

module Chart1Point5 =

    let from = (0, 0)
    let to' = (6, 4)
    let (arrowx, arrowy) = to'

    let options = Options(annotations = seq { 
        Annotation(showarrow = true, x = arrowx, y = arrowy, axref = "x", ayref = "y", ax = 0.0, ay = 0.0, arrowcolor = "red") 
    })

    [from; to'] 
    |> Chart.Line 
    |> Chart.WithOptions options
    |> Chart.Show

generates:

For the comments, I copied the xref and yref comments and modified them to say ax and ay since they should be operating the same (I hope). Hopefully that is ok, but if I need to do more research on how it works and provide a better comment I'm happy to do so. Let me know what is required.

I wasn't sure what kinds of tests, if any, might be required. If tests are required for new code. Let me know what I need to do. I wasn't sure.

Thanks!

[cartermp]

Thank you!