  We propose a fold change visualization that demonstrates a combination of
properties from log and linear plots of fold change. A useful fold change
visualization can exhibit: (1) readability, where fold change values are
recoverable from datapoint position; (2) proportionality, where fold change
values of the same direction are proportionally distant from the point of no
change; (3) symmetry, where positive and negative fold changes are equidistant
to the point of no change; and (4) high dynamic range, where datapoint values
are discernable across orders of magnitude. A linear visualization has
readability and partial proportionality but lacks high dynamic range and
symmetry (because negative direction fold changes are bound between [0, 1]
while positive are between [1, $\infty$]). Log plots of fold change have
partial readability, high dynamic range, and symmetry, but lack proportionality
because of the log transform. We outline a new transform and visualization,
named mirrored axis distortion of fold change (MAD-FC), that extends a linear
visualization of fold change data to exhibit readability, proportionality, and
symmetry (but still has the limited dynamic range of linear plots). We
illustrate the use of MAD-FC with biomedical data using various fold change
charts. We argue that MAD-FC plots may be a more useful visualization than log
or linear plots for applications that require a limited dynamic range
(approximately $\pm$2 orders of magnitude or $\pm$8 units in log2 space).
