Where to find historical short interest chart?

Is there any free data source for S&P 500 short interest data? For example, I want to be able to graph the level of short interest over time as a percent of float like here: http://ift.tt/2kkD3JX

Submitted February 07, 2017 at 12:19AM by pyrrhotechnologies http://ift.tt/2kBYN1P