A hyperbola. Made in KMPlot from the parametric function x=cosh(t), y=sinh(t). The graph was then mirrored around the y-axis in Inkscape to form the left-hand side. 

The hyperbola in the previous version of the file was made up of blobs, and not all of them were aligned correctly. I converted it to a high-resolution PNG using MediaWiki's own engine and then used Inkscape's "Trace Bitmap" function to convert it back to vector. I zoomed in and confirmed that the result is much closer to a real hyperbola than the previous version. (comment by Jesin)