ValueError if lines contain single elements

[fraseyboo]

So this is probably a niche case but when this code is presented with a 'line' comprised of a singular point _update_anchors() throws a value error as it can't surround itself with valid line segment points. In this case it'd probably make more sense to set xa, xb ya & yb to the position of the point itself. The rest of the code still works fine with this change and the label appears normally.

I changed the code to the following, the index is unnecessary but I kept it for readability.

        # Find the first line segment surrounding x
        for i, (xa, xb) in enumerate(zip(xdata[:-1], xdata[1:])):
            if min(xa, xb) <= x <= max(xa, xb):
                ya, yb = ydata[i], ydata[i + 1]
                break
        else:
            i = 0
            ya, yb = ydata[i], ydata[i]
            xa, xb = xdata[i], xdata[i]
            # raise ValueError("x label location is outside data range!")

The code still throws a warning about the anchor position but it still completes fine, there might be a more intelligent way to handle this though.