render_to_file should support rendering to png

`render_to_file` method can saves figures only in `svg` format even if we explicitly write `.png` at the end of file name.
I think it could be good idea to render such type of file as ending of filename suggest. That way is done in `matplotlib`.

Currently, we have `render_to_png` method which is responsible for exporting plot to png format and `render_to_file` which works only with svg format. In my opinion it's too general name, because png is also file. So I recommend changing name from `render_to_file` to `render_to_svg` and creating another method, `render_to_file`, which will support svg and png format as well.

Locate the existing render_to_file and render_to_png methods, then inspect how output formats are selected. Determine the expected API behavior for SVG and PNG, including the proposed rename, and verify that rendering a filename with each extension produces the corresponding format.

Written by the indexing model from the issue text.