[BioC] beadarray - bead level image plots

[Matt Ritchie]

Dear Krys,

> I was wondering what uncoloured rectangles signify in the image plots.

Uncoloured rectangles occur when there is no data to plot in that particular
region on the array surface.  There are a few reasons why this might occur.

1. If the region only contains outliers and BeadScan has been configured to
remove the outliers, then there will be no data to plot.

2. If the data is corrupt in that region (e.g. if log=TRUE and the region
only contains NAs -> nothing sensible to plot).

3. Another possibility, which only occurs when an image plot of the
residuals is made, is that the region only contains beads which haven't been
decoded properly (ProbeID = 0).  Bead types which have not been successfully
decoded are not summarised, so won't have a residual, and are thus ignored
for the image plot.

Best wishes,

Matt