plot_cells_heatmap: Plot heatmap of cell counts
In antonmalko/ettools: Utility functions for dealing with eye-tracking data

A generic function to plot heatmaps of cell counts. Shouldn't be used directly to plot counts produced by count_cells and related functions, since those counts may lack the data from certain subsets of the data, if those subsets lack the relevant kind of values (e.g. NAs or extreme values). Instead, plot_cells_count, or convenience wrappers around it should be called, since they take care of subsets with 0 observations by artificially adding these 0s back to the counts.

1	plot_cells_heatmap(dat, by, fill, limits = NULL, plot.name)

`dat`	data.frame returned by `count_NAs` or `count_extremes()`. It should have three columns: grouping variable (subj or item), name of the cell (region+fixationtype by default), number of observations in the cell
`by`	quosure. name of the grouping variable (normally, first column in `dat`)
`fill`	quosure. name of the column w/count of observations (normally, third column in `dat`)
`limits`	vector of two integers: lowest and highest N of observations possible in the cell to calibrate color coding. If NULL (default), the limits will be set automatically based on the data.
`plot.name`	character