RFC: consistent API for topomap plotting

[drammock]

In working on the API / implementation of .plot_topomap() for the new Spectrum class, I went to our existing API for inspiration. Turns out it's rather fragmented. Here's a table of the signatures for:

• Epochs.plot_psd_topomap()
• Evoked.plot_topomap()
• AverageTFR.plot_topomap()
• Covariance.plot_topomap()
• Projection.plot_topomap()
• the WIP Spectrum.plot_topomap()

...along with notes about what each parameter does.

Epochs	Evoked	AverageTFR	Covariance	Projection	Spectrum	affects
bands		fmin,fmax			bands	data: freqs to plot
ch_type	ch_type	ch_type	ch_type		ch_type	data: channel type to plot
normalize					normalize	data: divide each band by total power
agg_fun					agg_fun	data: how to aggregate over freq bins within band
dB					dB	data: log-scale
	scalings		scalings			data: V → μV etc
proj	proj		proj			data: projectors applied or not
"tmin,tmax"	times	tmin,tmax				data: which times get subplots
	average					data: window duration around times
	image_interp		image_interp	image_interp		dataviz: spatial interpolation
	extrapolate		extrapolate			dataviz: extrapolation style
	border		border	border		dataviz: extrapolation boundary value
	contours	contours	contours	contours		dataviz: contour lines
outlines	outlines	outlines	outlines	outlines	outlines	axes: head/skirt/etc
sphere	sphere	sphere	sphere	sphere	sphere	axes: head param
	sensors	sensors	sensors	sensors		axes: bool or MPL string like r+
	show_names	show_names	show_names			axes: sensor names
	mask		mask			axes: which sensors
	mask_params		mask_params			axes: style sensors
	time_unit					subplots: titles
	time_format					subplots: titles
	res	res	res	res		subplots: side length in pixels
	size	size	size	size		subplots: side length in inches
cmap	cmap	cmap	cmap	cmap	cmap	colormap
vlim	vmin,vmax	vmin,vmax	vmin,vmax	vlim	vlim	colormap: limits
	colorbar	colorbar	colorbar	colorbar		colorbar: on/off
cbar_fmt	cbar_fmt	cbar_fmt	cbar_fmt		cbar_fmt	colorbar: label
	units	unit	units			colorbar: label
axes	axes	axes	axes	axes	axes	figure: use preexisting axes
	title	title	title			figure: title
	nrows					figure: subplot layout
	ncols					figure: subplot layout
show	show	show	show	show	show	figure: show or not
n_jobs					n_jobs
verbose			verbose		verbose
			info	info		source of sensor info
bandwidth						EPOCHS ONLY (multitaper params)
adaptive						EPOCHS ONLY (multitaper params)
low_bias						EPOCHS ONLY (multitaper params)
normalization						EPOCHS ONLY (multitaper params)
		baseline				TFR ONLY (baselining)
		mode				TFR ONLY (baselining)
			noise_cov			COVARIANCE ONLY (whitener)

As you can see, there are many inconsistencies, some of them rather egregious (e.g., 'unit' vs 'units' for the same functionality). Some inconsistency is to be expected since some of these methods are doing different things (e.g., Evoked.plot_topomap() just plots the average signal value and does not offer any frequency-specific / power plotting).

I would like to take this opportunity to standardize the plot_topomap APIs to the extent possible. I know we're often a bit lax about backward compatibility when it comes to plotting functions, but bringing these all into some semblance of alignment will be a pretty big API change I think. One possible approach to consider is coming up with a new name and letting the old method names hang around for a couple of releases (with deprecation warnings). Possibilities:

• plot_field is already used for visualizing the field projected onto a 3D surface (scalp/helmet)
• plot_topography is the most neutral choice I can think of, but also vague
• plot_scalp_topography is long but is evocative of the head diagram, so maybe good?

Would love to hear others' thoughts on this.