What is "scaled mean expression" in dot plots?

"scaled mean expression" gives a visual indicator of where each cell type's mean gene expression lands relative to the smallest and largest mean gene expression for the gene of interest across the cell types shown in the dot plot.

Scenario: in cell type x the mean gene expression of gene “random” is 2, and in cell type y the mean gene expression of gene “random” is 1

Example 1

IF the dot plot has ONLY cell types X and Y, because X has the largest value, the "1" end of the scaled mean expression bar  corresponds to a mean gene expression value of 2 (ie the dot for cell type X will be red). While Y, having the smallest value, means the "0" end of the scaled mean expression corresponds to a mean gene expression value of 1 (ie the dot for cell type Y will be blue).

0 —------0.5—------1
|                             | 
cell type                cell type
Y(blue)                  X(red)

Example 2

If instead the dot plot has more cell types, for example, cell type A has mean gene expression of 4 for the gene of interest and cell type B has mean gene expression of 0 for the gene of interest, then A and B would be set to the "1" and "0" ends of the scaled mean expression, respectively.

        cell type Y(pinkish-red)
        |
0-----^-----0.5-----v-----1
|                          |       |
cell type B(blue) |       cell type A(red)
                          cell type X(purplish)

In this second example, cell types X and Y are plotted relative to the smallest mean gene expression (cell type B) and the largest mean gene expression (cell type A).

Note: when you hover over any particular dot in the dot plot, a box will appear - the number in the top left of the hover over box shows the actual mean gene expression value for that dot (the bottom line in the box shows the percentage of cells that expressed the gene of interest)