Dotter is a graphical dotplot program for detailed comparison of
two sequences.  Here, every residue in one sequence is compared to
every residue in the other sequence. The first sequence runs along
the x-axis and the second sequence along the y-axis. In regions
where the two sequences are similar to each other, a row of high
scores will run diagonally across the dot matrix. If you're comparing
a sequence against itself to find internal repeats, you'll notice
that the main diagonal scores maximally, since it's the 100% perfect
self-match.

To make the score matrix more intelligible, the pairwise scores are
averaged over a sliding window which runs diagonally. The averaged
score matrix forms a three-dimensional landscape, with the two
sequences in two dimensions and the height of the peaks in the
third. This landscape is projected onto two dimensions by aid of
greyscales - the darker grey of a peak, the higher it is.

Dotter provides a tool to explore the visual appearance of this
landscape, as well as a tool to examine the sequence alignment it
represents.

WWW: http://sonnhammer.sbc.su.se/Dotter.html