plotter: NewPolygon should take an interface rather than a list of interfaces

Right now, plotter.NewPolygon takes `...XYer` as an input. However, it seems like it is difficult to actually get data in that format.

Would it make sense to add an interface something like:

```go
XYers interface{
Len() int
LenAt(int) int
XY(int, int) (x, y float64)
}
```

That might be easier to implement.