.plot method for RoIs

[willGraham01]

It is currently possible to plot RoIs by interating over their coordinates and passing this to a matplotlib function, like in the example in #440. However as highlighted in the PR discussion:

One other food for thought; in this example I plot the region of interest manually, using the coordinates that are stored on the object. But it probably wouldn't be too much effort (and quite nice for users!) to have a .plot method for Regions of Interest. It might also be useful for us given that we'll have to "plot" the regions in napari anyway, eventually!

That's a great idea! Thogh the "plot" method would be a maptlotlib wrapper like the ones we already have right? The method for showing the ROIs in napari has to be different.

Originally posted by @niksirbi in #440 (comment)

Within the BaseRegionOfInterest class, all we'd need is a method like

def plot(self, ax, **matplotlib_kwargs) -> None:
    # self.coordinates iterates over (x,y) pairs of vertices that define the region,
    # so unpacking this into an array of x and array of y positions should be easy.
    # Then pass to the matplotlib method.

There are some differences between considering segments / lines (1D) and polygons (2D). We might want to fill the region occupied by a polygon for example, and these polygons might have holes too. For 1D objects I think the convention is fairly clear-cut - we can even just use matplotlib.pyplot.plot to draw the regions right away.

[niksirbi]

I'd favor "filled" regions for PolygonOfInterest and just "lines" for LineOfInterest (including loops). I thinks that's what you mean as well, right?

[willGraham01]

I'd favor "filled" regions for PolygonOfInterest and just "lines" for LineOfInterest (including loops). I thinks that's what you mean as well, right?

Yeah that was my intended meaning.

I just wanted to flag in the issue that it means that we might need subclass-specific implementations of .plot rather than one implementation in the base class. It won't be the same matplotlib method that is used to create both plots; plot for the lines and most likely fill for the polygons, plus some wriggling to deal with holes in the polygons.