Sequence label in matplotilb does not work well with window_size option

When plotting:
```bash
./dotplot.py --fasta 1.fa 2.fa --gui
```
and
```bash
./dotplot.py --fasta 1.fa 2.fa --window_size 10 --gui
```

in both cases we really consider all residues/nucleotides etc, so full sequence should be shown in both cases. In the second case, the sequence is trimmed to the dimensions of plotted matrix, which shouldn't be the case.