The Layout Placeholders tab of the Working Illustration gives a bird's eye view of the layout of the current illustration. Instead of displaying data, though, test tube icons are shown in order to save space.
Within Layout Placeholders, as is the case in the Working Illustration, you can click and drag column or row headings to change their display order. The test tube icons themselves cannot be dragged.
(click and drag to change plot layout is supported in the layout placeholders tab as well as in the illustration tab)
The Layout Placeholders tab is very useful for evaluating potential problems with Sample Tags. When each test tube icon has its own square, as is the case in the layout above, all is well.
Consider the example below, however, where a sample that is supposed to be in the LPS Sample Tag has accidentally been assigned to the Unstim Sample Tag:
(A missing sample in LPS and multiple samples in Unstim, as seen in the layout placeholders tab)
Indeed, visiting the tagging page for these Sample Tags shows the problem:
(after reviewing layout placeholders, a problem with Sample Tags can be fixed easily)