Consider adding defaults for override_x_max and override_y_max

[Bergam0t]

At present, override_x_max and override_y_max default to None, which means plotly tries to set up the extent of the plots itself.

This may be a sensible behaviour, but particularly if going on to using an image, it can make accurate placement almost impossible.

Would it be worth setting a default? Or, alternatively, if providing a background image, should defaults be set then? Is this going to be confusing behaviour?