3 test failures

I tried running the nosetests, and got failures. I put in a pull request to take care of one source, but three remain:

symlog2 svg
image_interp pdf
tight_layout5 svg

I have only looked into the last of these, which has nothing to do directly with tight_layout; on my system, the svg file is being generated with interpolation, even though the call to imshow has interpolation="none". I don't know what is going on. When I simply make an image with that kwarg and save to svg, it's fine. But it is not working that way in the test.