I checked through every Mac option on this list: http://connectedresearchers.com/graph-digitizer-comparison-16-ways-to-digitize-your-data/ and only WebPlotDigitizer worked reliably. Since it’s web based, it works on pretty much any platform. Its automatic curve tracing feature works quite well, but one caveat is that it doesn’t work with transparent images (fails to find the curve) – lot of head scratching to figure that one out!

See the following example of how well it digitizes, even with experimental noise.

Original:

Digitized:

Pretty good, I’d say.

 

Update May 14th 2016
At my suggestion, the author of WebPlotDigitizer kindly fixed the issue with transparent backgrounds.