Comment 3 for bug 196291

In the meantime, try the following fix, or anything similar to it.
http://linuxtnt.wordpress.com/2008/04/19/fix-for-gnuplots-buggy-svg-output/
Replace what currentColor with the color defined in "color:" property.

By the way, Opera 9.51 and Mozilla Firefox 3 seems to render them fine as they are. According to the site I mentioned, the currentColor is white so everything (though when I set the background color to something else, the lines still don't appear) dissapears from sight, although still existing. Either Inkscape's interpretation is incorrect, or it's too strict, I don't know much about SVG standard though.

To add, plot points are also lost. To make them reappear, you have to change the fill color as well.