Length legends' SVG exporting logic doesn't account for long titles

[fedarko]

Minor bug: when the length of a legend title (usually, the corresponding metadata field used to encode lengths in a barplot layer) is longer than the longest rows within the legend, this is not accounted for. This bug results in the exported SVG legends looking like:

This should be pretty simple to fix, fortunately. This is coming up as a problem for #322.