Maps with 8-bit unsigned integer data should probably not set norm to None

[ayshih]

There is very old code (from #1432) that sets the norm to None for maps with 8-bit unsigned integer data (uint8, i.e., 0 to 255):

sunpy/sunpy/map/mapbase.py

Lines 236 to 237 in 5bffcc2

	if self.dtype == np.uint8:
	norm = None

Presumably this is done because 8-bit unsigned integer data maps directly to the 8-bit color scale for matplotlib. However, this causes later code (written much later) to fail because it assumes that the norm keyword exists only if it is actually a normalizer, e.g., in plot():

sunpy/sunpy/map/mapbase.py

Lines 2529 to 2532 in 5bffcc2

	if 'norm' in imshow_args:
	norm = imshow_args['norm']
	if 'vmin' in imshow_args:
	if norm.vmin is not None:

So, possible solutions include:

• Don't special-case the setting of norm for uint8 data (i.e., treat it the same as any other data type)
• Don't set norm at all for uint8 data (i.e., norm will not be set in plot_settings)
• Add special code in plot() specific to uint8 data

Choosing between the options depends on exactly how we would like uint8 data to be visualized.