Customizations to charts persist unexpectedly

Motivated by http://stackoverflow.com/questions/31305580/fsharpchart-two-scales-on-same-graphic.

I assumed that "customizations" to chart objects were done in a non-mutating way, i.e. a fresh new chart was created with the specified customization, and the original was left unchanged.

But it seems that many customizations are "sticky," as if the original chart object has been mutated. This feels like a bug.

``` fsharp
#load "../packages/FSharp.Charting.0.90.10/FSharp.Charting.fsx"

open FSharp.Charting
open System.Windows.Forms.DataVisualization.Charting

// displays, no title. good.
let chart =
[1..10]
|> List.map (fun n -> (n, n))
|> Chart.Line ;;

// displays, with title. good.
chart |> Chart.WithTitle(Text = "Test") ;;

// displays, *with* title. why?
chart ;;
```