To add a gene name, use the text track.
You can position the labels to coincide with the tick labels position, if you like.
The SVG canvas size is set to the same number of pixels as the PNG image size. This is set by the 'radius' parameter in pixels, which defines the radius of the inscribed circle -- so 1/2 of the image width. Most software will interpret the physical size of an SVG canvas defined in pixels using 72 ppi conversion.